The watch has a lipo cut off function....so you won't damage the battery by running the watch battery to 0%.....as it won't in realty be at zero....it will have a safe amount of volts. Best to charge your battery when the watch is very low on power....as you damage a battery everytime you charge a battery slightly. To store a lipo battery....is best at half charge.... If you don't charge the battery for a long time....say some months..and it was flat when stored...that could also damage a lipo.
The watches 0% should be around 3.2v battery power left. 3.0 volts per cell under load is a low as you should go before battery damage..( although I use lipo alarms that I program to cut off at 3.2 per cell balanced )..that's why its safe to drain the battery to 0% on the watch with its safety battery voltage cut off to above 3v under load. I'd like to know the c rating of the battery used in our watches..
Someone wrote it here already, just repeating. Count with me: With 5-7 hours of gps activities per week and full day/night wear I need to charge like each 5-6 days. With average 400 cycles we get to 2000 days, which is more than 5 years, which I hope I am going to have already Fenix 8 plus :)