How consistent are Watch movements?

Yep. Those amongst us that harp on quality of movement do so for this reason. That said, one company may have better tolerances and regulation on the same movement. Basic Seiko movements notorious for losing or gaining time. Sometimes you get one that is +/- a couple seconds, sometimes it 45s. 

What's the spd +/- on the Seiko? 10-20 a day right? So yeah, an adjustment every 3-6 days would be about right. 

Based on the below, an adjustment every day is within spec on some.

As far as the power reserve, wearing it a week straight might not fully wind it depending on what the activity level of the wearer. Are you sure it's fully wound when testing?

You can regulate the watches, but you will find quite a bit if variation between different watch positions. The rates will vary by a few seconds, so simply you taking a day off vs. being at the office can trigger a variance. For that reason I really don't worry about anything under +/- 5 spd.

Yeah it's an 4R36 movement and it's starting to annoy me a bit that it is in reality very inaccurate, the Hamilton is fine and very accurate but occasionally it stops a bit too soon. I do want to buy another watch and Seiko just destroy everyone else in the sub 1000 price bracket for dials (which is what I like the most about a watch) but the inaccuracy is putting me off them a little cause having to constantly adjust them is irritating and also means I can't fully rely on it to tell the time.

This is why I am put off them a bit cause it doesn’t need to be Quartz accurate but so that I don't need to adjust it all the time so I might go Swiss for my next purchase.

If you want a consistent mechanical watch movement that you won't have to worry about adjusting for weeks, get a spring drive from Seiko or Grand Seiko. With spring drive you get a self winding watch that's powered by a main spring, but is also regulated by a quartz oscillator. 

Even the basic 5R65 spring drive movement from Seiko is very accurate and very consistent in my experience. The one in my collection went for over two months before I adjusted it and it was only running about 4 seconds fast at that point. I haven't allowed the power reserve to run down, but it seems to wind pretty efficiently. Even on lazy weekends where I spend most of my time lounging around, the needle on my reserve indicator has barely moved.

Thankyou for the spring drive explanation! I have wondered exactly what that meant! Spring drive movements are not found near the budget end of seiko watches though are they?

I think it's all a matter of how much accuracy meet your expectations.  Obviously cheaper movements can be off as much as 30 seconds/day and still meet their manufacturer specs.  So being off by a minute every 3rd day wouldn't be necessarily alarming.  Although my experience is most affordable watches I have seem to do much better than that to be honest.  So maybe a little service and regulating and wouldn't surprise me if you can get them to be much better than that. 

Of course if it's a watch that claims chronometer grade you're looking at closer to 5 secs/day.  So yeah if you're off by a minute every couple days, might be time for servicing.  

In terms of power reserve that to be honest I guess depends on how active you are when you use it.  Honestly if you're wearing it a week non stop, I also would expect it to be close to fully wound and get close to that 80 hr reserve.  

If you really are a stickler for accuracy, quartz is the way to go though and power is well a battery replacement away.  Mechanical watches are little engineering wonders but they do have their compromises.  

They are not particularly affordable. Some cost less than others, but expect to pay anywhere from 2,000 to 3,000 USD on the very low end and you may have to buy pre-owned to find something in that range depending on the model.

Just keep in mind that it will be a function of price, even if you go Swiss. A base ETA 2824-2, or whatever the brand named a variant, will not come close to what you are expecting, either. This changes with higher-tiered movements that are regulated to different standards. Similar to what @UnholiestJedi mentioned, the cheap $30 nh35 movement on my wrist right now is about 5 spd fast, but it spent some quality time on a timegrapher after I got it. COSC-certified watches have an average accuracy of -4/+6 seconds per day, plus some additional temperature and position requirements, but they are in turn also using average and not individual readings.

What I'm really trying to say is: Exceeding the accuracy of a well-regulated base movement won't be cheap.

The Swiss are usually all about accuracy whereas Japanese accuracy is by price point. Chinese movements are good if the Swiss service and regulate them first. Get a Tissot Powermatic 80 with the altered ETA2824 movement.

The CITIZEN Caliber 0100 Limited Edition   (+- 1 second / year)  Not much adjusting needed here. 😂

I have only had the Seiko for about a year and a bit and this has been an issue for about 6 months and I did buy it brand new, what is also ironic is that the Seiko cost me more but the movement is shit by comparison 😂

I guess my point is it kinda depends on what the manufacturer will guarantee whether it's a Swiss or Japanese movement.

Obviously if it's a COSC certified Swiss watch you expect it to be accurate. Likewise if it's a well regulated Japanese movement that claims Chronometer grade it's probably going to be accurate. A lot of microbrands will use regulated NH or Miyota movements and they can easily match a Swiss COSC certified movement for accuracy.

But if there's no certification then it's really luck of the draw on accuracy.  As long as within spec you're watch is technically not malfunctioning nor does it mean every watch with that movement will have the same accuracy. So I wouldn't discount the movement as a junk movement based on that.  Even say a low grade Swiss Sellita  SW200 can be off by 30 sec per day and still meet spec, much like a Seiko 4R movement. 

Maybe I've been lucky but none of my watches meet the upper or lower limits of their specs, at least so far.  I literally have a Seiko 5 that I'm sure is easily under 3 sec a day, consistently.  But that's just luck.  I don't expect all of my Seikos to perform that well and neither does Seiko haha.  I have watches with higher grade movements that don't match that Seiko 5.  And I have other watches with that same 4R or NH movement that are obviously not that accurate.  But they are all within spec so it's ok.  Like I said luck of the draw.  

If there is one difference I do see in terms of quality between Swiss and Japanese movements, it's not so much accuracy but refinement.  I feel like when adjusting time, hand winding etc...Swiss movements tend to feel mechanically smoother and more refined.  Japanese movements kinda depends on who worked on it.  I feel microbrands do a better job with Seiko movements than Seiko themselves.  I have some that are great but others definitely work functionally well but feel cheaper or like you have to baby it or it will break. But all my Swiss made watches all feel very refined when using the crown and functions even the most affordable ones.  There is something to that feeling of confidence in build quality.  Cant say that about all my watches with Japanese movements.    

If you want accuracy, buy a Tudor ( in house movement ) 

Or one of those METAS certified watches. Master chronometer rules, yo.