3 years, 918 cycles: time for retirement & replacement
15 - 30 minutos
I always enjoyed the freedom from the cables given by a long lasting battery: with little more than one hour of video playing, and much less than one hour of XCoding, I couldn't get very far from a plug. My freedom was in danger.
It took more to remove the screws securing the lower case than actually replacing the battery: my little sister could have done it - as easy as playing with Lego.
Make sure you have the right screwdriver for the battery: mine just looked like it could be small enough to fit. Actually, I had to use pliers and patience: no damage and more adrenaline.