How long does it take to replace the blend door actuator?

Usually takes about a half hour. Remove the trim panel under the glove box and the actuator is just to the left of the blower motor on the side of the HVAC case unplug the connector, remove the screws and the actuator. Install the new one, be sure it's clocked right.
Now you need to calibrate the replacement.
Start with the key off, remove the HVAC fuse (# 12) for 20 seconds. (You can even pull it before you start replacement if you like) After the time, put the fuse back in. Start the engine and let it run for at least a minute to calibrate, test the controls.
FYI all of the actuators are the same part number GM 22877649