5 - 10 minutos
My wife's 13" MacBook Pro was only lasting an hour and said "replace battery" when you clicked on the menubar icon. A new battery was needed and iFixit had it.
Since I was ordering the MacBook battery anyway I also added a battery for my iPhone 4. Now with iOS 5 the battery life wasn't what it used to be (some of that is due to bugs acknowledged by Apple).
The MacBook replacement was a breeze. 10-15 minutes tops. Just unscrew the bottom cover, then a couple of screws holding the battery in place. Insert new battery and put cover back on.
The iPhone 4 battery was more difficult. There is a part that comes loose during removal of the old battery. The screw holding the battery in goes through this part and holds it in place. You must clean this part or not get oils from your fingers on it. Most difficult part was getting this in the right position to put the screw through it.
Use tweezers to hold the little piece in place. Then you have a good grip and don't get the thing dirty and risk poor cell reception.