20 - 30 minuten
I got a used iphone 3GS on eBay and it worked fine but the battery didn't last a few hours until it needed recharged.
It went fairly smoothly. no surprises. The directions on the site were spot on and I had the new battery in the phone in about a half hour. Re-assembling was a bit of a pain getting some of the connectors back on. Just because they are SO small. My bifocals didn't help much!
It is not as easy as an android phone to replace the battery. You will be taking the whole phone apart to replace the battery. You will have to use some force on certain parts (removing the main board) and a lot of force on the battery to loosen the adhesive. If you are moderately handy, you should have no problem.
If you are not comfortable with VERY small parts - DON'T DO IT!
Make sure you put the screws on something (I used a piece of paper with a drawing of the phone) to put them back in the correct place.
Make sure it is a place where you won't be disturbed for a little bit with a clean, clear workspace and all should go well. A good bright light is also a plus. A magnifier light would be ideal, I didn't have one.
The scary part is thinking you will break something during the procedure. You might, so READ the directions for each step until you understand what you need to do BEFORE you do it. The pictures are great but do not tell you the whole story. Read the direction before ordering to make sure it is something you CAN do.