Easy fix where Apple would overcharge and refuse.
15 - 45 minuten
Battery life was dropping like a rock with practically no use. Apple tested the battery and said it was "healthy" and refused to change it out. I had my computer completely reformatted in the store and without any additional software installed, the charge started dropping like a rock again. "Nope, it's healthy, my test that ran in the background by itself for 2 minutes proves it - we don't need to and will not do a full stress test since it will say the same thing."
Well I decided since my laptop wasn't covered any longer by AppleCare, and Apple was refusing to actually help, I'd just do it myself. Ordered the battery and tools from iFixIt via the free shipping option, and they arrived 2 days later via USPS. Actual battery-changing operation took less than 10 minutes, just being careful to keep track of the screws.
Follow the instructions on your device's associated iFixIt guide. Also, pay attention to what screwdriver you're using. I accidentally used the 5-point screw to screw in the 6-point screws. Kind of stripped the screwdriver head. Oops.
MacBook Air 11" (Mid 2011-Early 2015) Replacement Battery
T5 Torx Screwdriver
P5 Pentalobe Screwdriver Retina MacBook Pro and Air