A Lion ate my fans
20 - 30 minuten
After upgrading to Lion my Wife's 15" MBP's fans decided they were going to continually run. It eventually got to the point where the right side fan was buzzing and there was soot outside of the vent from the fan.
Went fine. Before purchasing the fan I had disassembled the unit to try and look and see if it was debris in the fan. Second go around was MUCH faster and taking it apart of replacing the fan.
Don't upgrade to Lion until they figure out the fan/heat issues. If you look on the Interwebs you can see that this is NOT an isolated incident. Snow Leopard had none of these problems.