I've had this mouse for about 4-months. I'd never thought I could fall in love with a mouse.
I usually place it on the charger every night (if I remember) because I use it a lot. There have been plenty of times when I've only left it on for 15 minutes. I don't wait until I drain the batteries (then I'd be without a mouse). I still use the original set of batteries, and they still hold a charge for several days with fairly heavy use.
I wouldn't let how often you charge ,or for how long you charge them, concern you too much. Nickel Hybrid batteries are much more tolerant and cheap to replace if you have to.