To what percentage should the watch be charged?

Similar to what G141592653589793238 mentioned. This one from Battery University.

BU-415: How to Charge and When to Charge? - Battery University

Li-ion is maintenance-free and the battery lasts longest when operating between 30 and 80 percent SoC.

Personally I just put it on charge when I'm in the shower or bath, when it's below 50%. 30 minutes of charge every few days seems enough to get it to about 80-90%. If I was expecting to be doing a very long activity soon afterwards I'd let it charge fully.

Maybe, but still...

You use it as if it's limited (only use 50% of the capacity) to prevent it from becoming limited in the future? As opposed to using it normally, full capacity, which might in turn make it limited in the future?

I just don't really get it... Just use it as it is, enjoy battery life as it is and don't bother with it too much. Charge when convenient, unplug when convenient.

So far battery have never been an issue for me (but I mostly have devices for ~3 years max).

I don't know if I'm confused but it wouldn't be 50%, in fact it says it charges it to 80-90%. The idea of not always charging to 100% is that in that range from 80 to 100% is where the batteries overheat the most and that is where the most damage could be done. I understand that if you always keep a scheme of using it from 0 to 100 would be the worst scenario for the battery and that shows the tests. Now, I agree that they will still die at some point, but maybe that would give you some extra weeks, months or years? It's not a complicated thing to do and you know you're not continually pushing your battery to the limit when you don't need to.

Yeah, but you also don't let it get below 20%? So not letting it get below 20 and not charge above 80 = 60% of the capacity ;)

But I charge it when it gets to 10-ish en charge it to 100. Perhaps if I plan to use the device/watch for 5 years+ it might matter but so far I've never had any issues.

I do know the theory behind, ut my theory is:

Scenario 1:

You charge when the battery reaches 20 and up to 80%, never using more then 60% of the capacity and instead of 4 it might last you 6 years.

Scenario 2:

I charge it from 10 to 100, using 90%... Perhaps after 3 years the battery starts to perform worse, well, if it only drops to half of it's original performance I still almost get 50% after enjoying almost max performance for 3 years ;)

Anyway, to each their own and all fine of course... I honestly just can't be bothered with it and charge when needed and just let it lie there. 

Thanks, now I understand your calculation. It's relative talking about of "capacity of use" because it will depend on the use of each one. If for the daily routine of a user, recording activities and all, wearing his watch with 70% battery is enough, maybe when he comes back from the day it will be a comfortable routine to charge it from 40 to 80-90% while taking a shower and have it ready for the next day. You lost nothing, gained nothing in terms of capacity and have it ready for the whole next day. On the other hand, if you had left it at 40%, surely halfway through the next day you would have to put it on charge to be able to complete the day. There are as many scenarios as there are users. I don't know if anyone has done an analysis of how much battery life can be extended in this way. In my experience also after 18 months of use they start to decline, some brands even before.
I agree with you, there is no need to worry too much, but if it does not cost anything and coincides with your routine you can follow this scheme that you know will be somewhat better for the battery and other components by not raising so much temperature. Many times it has happened to me that it turns off because I have nowhere to charge it or charge it 100% because I know I am going to need the battery to the maximum, it's not a problem also.