I have the Wireless keyboard, Magic Mouse and Trackpad. When I first got my iMac in 2009, I went and purchased some rechargable batteries to use in the mouse, I found that they would work for a few months and would need a recharge, at this point there were a lot of people debating between regular AAs and rechargables.
I switched to a regular AA and found that it lasted significantly longer than the rechargables.
I doubt any rechargable battery will last for 10 years with a number of cycling. If you're getting to a point where the batteries are dying within a couple of days, you've reach the end of the life for those batteries and need to purchase new ones.
The keyboard, magic mouse and keypad all automatically go into sleep mode after some inactivity and don't need to be turned off. If you are going to be away from your computer for a week or more, then by all means turn them off. But just for overnight or a day..there's no need.
I currently use generic AA's from my local wholesale club in my keyboard and mouse and get about 10 months or so on the keyboard and easily 6 months on the mouse.
Now, the battery life are obviously affected by your usage. I use my computer for about 1 hour each weekday about 4 hours over the weekend. So approx 10 hours a week, if your usage is higher, your battery life (especially for the mouse) will be shorter..