A mouse has 3 rooms to go into.

If it goes into room 1 it will find the cheese after 3 minutes.

If it goes into room 2 it will look for cheese for 4 minutes,
won't find it, and will go out.

If it goes into room 3 it will look for cheese for 5 minutes,
won't find it, and will go out.

The mouse will not remember that it was in rooms 2 and 3 after it
goes out of them, and it will continue going in and out until it
finds the cheese. (It can go into the same room again and again.)

What is the average time for the mouse to find cheese?