5 - 30 seconds
My MacBook keyboard after seven years, lots of typing and one errant berries smoothie later, was missing a few keys and had totally lost the use of its delete key. But the final straw was an e key that had decided to either not work or depress itself continuously mid-typing likeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee this.
Took about an hour and a half I think, because I was very gentle and didn't want to damage anything. The yellow electrician's tape was great for replacing the original dusty old ones that had lost their adhesiveness.
Hmm, check all steps carefully before replacing the case - it's tiring to remove all those screws again.