10 - 15 minuten
Noticed one day that the trackpad wasn't working as well for me when I used my left hand. Having had this problem before on a different Mac, I suspected a swollen battery, which presses up against the trackpad from the inside. Indeed, looking closely, I could see that it was no longer inset, and one corner was raised.
Opened up the Mac (after struggling with the tiny phillips screws) and was dismayed to see that the battery was held down by tri-wing screws! Called Apple to make a genius appointment, but they refused to tell me in advance if they had the battery in stock. I didn't have the time to go there and possibly be told to come back later to pick up the laptop. (How could Apple not have stock information on replacement parts!) Ordered the tools and replacement battery from iFixit, despite the (slight) chance I might have talked the genius into replacing the battery for free (I hear they sometimes do, since exploding batteries are bad PR...).
Once the battery and tools arrived, the replacement took about 5 minutes!
From the moment you finally realize that the trackpad doesn't quite "feel" right to when the battery swells up enough to make the trackpad unusable you have a week or two, so better hurry to get the replacement battery in quickly. Putting the laptop to sleep when not used, unplugging the power overnight, etc., will extend that time a bit, as it shrinks a little when not used.