5 - 30 minuten
I've been using my iPhone 4 heavily nearly every single day for the last 5.3 years! It blows my mind that the battery was even operable still. But operable and helpful are two different things. It only had maybe 1-2 hours of usable battery life, so it was definitely time for a new one.
Ordered the parts, got them pretty quickly. It came with the tools like promised. I wasn't sure if the battery cable would be pre-bent (had heard they weren't), but mine was when it arrived (simply saving me a step). It was a total of 3 Philips screws; the 2 on the bottom, and 1 inside for the battery connector. In total maybe 5-10 minutes of time. Biggest trouble was the adhesive which was stuck well and good after 5+ years! The old battery ended up being pretty marred after removal because of the force needed (since using the lift tab would have snapped it).
When you finish the installation of the battery, be forgiving on the phone. Don't worry if it doesn't start up immediately. Put it on a charger cord and keep watch over it for 15-30 minutes (don't leave it unattended in case fire risk like a bad battery). Mine didn't turn on right away and I was a little sad, but after 5-10 minutes it finally charged up enough and the phone booted up.
Otherwise, go slowly with the whole process. Slow is smooth, and smooth is fast. Be careful with the adhesive, take your time and listen for the glue releasing etc. Be extra careful to capture your screws and parts, you probably won't find them if you drop them.
And... you got this! This is a very easy repair, don't psych yourself out, it's worth it.