5 - 30 Minuten
The battery life on my iPhone 4 was getting bad....dying in 4 hours under use, and 8-12 if just on standby.
To replace the battery was pretty darned simple...except the pentalobe screws are darned hard to see. I couldn't get them back in. Fortunately, I bought the "liberation kit" which gave me standard phillips head screws which went back in with minimal fuss.
The only hard part was removing the battery from the main board --- the connector is a little "odd" -- but it came out and went back on with minimal fuss --- anyone with a reasonable mechanical aptitude will have no problem with this part. Just don't force anything, and go by feel. You'll be fine.
I was also surprised at how much force it took to remove the battery itself. You aren't pulling on a tab on the batter, theres a tab on the phone that acts as a lever. Just pull...with confidence. That battery will remove with some force.
Advise: Good lighting, good magnification are important. Lay down a black work surface if you have it. It will make finding screws easier.
Time: From beginning to end, it took maybe 15 minutes including looking for my old stand by jewelers screwdrivers and magnifying glass.