I don't turn off my Magic Mouse at night - it should go into sleep mode automatically.
I'm still on my original set of alkaline batteries with the Magic Mouse I recently got with my 27" iMac (just over 3wks now) so I can't say for sure how battery life is yet with the new MM. But I have heard reports the battery life isn't so great. however...
FYI for a few year's I've been using the predecessor - the wireless Mighty Mouse. here's what I do - I use Sanyo Eneloop rechargeable batteries, these are NiMH batteries with extremely low self-discharge rate (they say 85% charge retained after a year on the shelf). by comparison regular NiMH cells discharge a percent or so per day.
I have 1 pair in the mouse and another charged pair always at the ready in my desk to swap out when needed. It's not practical to do this with regular NiMH cells, I've found them to lose their charge too quickly even when the mouse is asleep, and the spare set in my desk would always be low on charge once they were needed. Since switching to Eneloops I get many weeks of life out of them before recharging and the spare set are always charged up. I recommend anyone with a bluetooth mouse to give them a try
it's possible that the new Magic Mouse is more power hungry than the old mouse, with that big touch sensitive surface. I'll see how it compares when my original alkalines die, but I'm thinking this 2 pair of Eneloop system will continue to work fine for me.