25 - 45 minutes
My son (age 7) really loves music, so I decided to get him an mp3 player. I wanted something without a touchscreen that was durable and could only really be used for music and audiobooks. The old iPod Mini was a perfect match to these requirements. I found one on eBay for 10 bucks. It had a scuffed case and a bad battery, but otherwise in working order. iFixit to the rescue with the battery kit!
The repair went pretty much as described in the iFixit tutorial. The iPod casing was already scuffed and scratched, so I didn't feel the need to be /quite/ as careful removing the top and bottom panels as I ought to have been. It took less than 10 minutes to perform the swap.
IIRC, the tutorial shows a small screwdriver being used to remove the top and bottom plastic panels. If I did this again, I would use something thinner. The guide mentions this, but I will also stress that it is /very/ easy to damage the plastic, as well as the aluminum housing.
Also, make sure the internals are clean when you reinsert them into the housing! I ended up with a tiny stray hair stuck across the screen, which I didn't notice until all was reassembled.