So, in last 10 days, I've done about 4 pool swims, 2 bike rides and about 4 outdoor runs. I kept a log. Based on my experience, I see the following with battery loss:
Run/bike (gps and HR on/ phone and backlight off): 11-13%/ hour (or 8 hours total)
Watch (standard watchface/ HR off/ gesture backlight off / phone on every so often to sync/tap for back light every so often): 9-10%/24 hrs (or about 10-11 days total)
Swim: ** Although VA3 was perfect for counting lengths, I saw huge battery loss (like 20%/hour) even though HR/phone were off. Backlight kept coming on even though I thought I turned it off and watch kept "swiping" to various screens even though I turned on autolock for activities). Something weird was going on because swims also got recorded but for 6 hours in advance -- i.e., I swam at 12/18 at 6:30pm and activity appeared in app as 12/19 at 12:30 am. All non-swim activities got recorded with correct time.
I wish it was better and I think Garmin misleads with advertised 13 hours of gps, but I can live with this.
The only thing that kind of stinks is you have to be mindful of what's on/off.