Is it better for Pebble Time 2 battery endurance to regularly charge the watch to full (for example, daily while showering), or to only recharge when the watch warns that the battery is low, or some other frequency? Or perhaps it makes no difference?
(I generally only recharge my Pebble Time Steel when it is reporting around 20-25% or if the watch warns battery is low, and it still lasts for around a week after a decade of use.)
As usual for lithium, best is to keep it between 20-80%. Keeping it fully charged causes early degradation.
Given the battery life of the watch (2 weeks on average), I would not be concerned about battery endurance. If you really abuse the battery, leaving it on the charger all night every night, that would reduce the lifespan to 500-ish charge cycles (not charging sessions, but 0-100 charge cycles). Assuming you use 10% per day (10-day battery life, very safe estimate), that would take 10 days to do 1 charge cycle, and 5000 days to burn through the battery entirely. That’s over 13.5 years.
If you plan on using the same watch for more than 13 years, good on you.
Just set a schedule to charge it up once a week, maybe as part of a weekly getting-ready routine. Should be good to go.
I mean that’s about what we had to wait to be able to replace ours 
I do not have a Pebble Time 2 yet, but is there a way to limit the charge of the PT2 to 80% ?
Not currently. It’s being looked at, but low priority