Until you fully cycle the battery 2-3 times, the phone does not accurately give the proper % charge, even after a few cycles, its not perfect. To get a better idea of your battery level, us the widget called "Battery Left Widget". The longer it runs, the better the accuracy because it does much more then just give a percent based on battery voltage.
On the standard battery (1300mAh) you will probably get about 10-12hrs of good use out of it. I just recently got myself a 1750mAh battery and I got probably 16-18hrs of the same use on it. I have only cycled the battery once so far and based on the included info with the battery, I will get even better performance after a few charges.
If your looking to get a full 24 hours out of your phone, get the 1750mAh battery. If you need a full weekend, go for the 3500mAh but it will add some bulk. Otherwise, expect to have to plug it in at least once a day.
If you don't already know, you can use a standard mini usb plug to charge it. If you sit in front of a computer all day, you can just have it plugged in there.