Should I switch when monty hall shows a door?

There is a generalization of the original problem to n doors: in the first step, you choose a door. The game host then opens some other door that's a loser. If you want, you may then switch your allegiance to another door. The game host will then open an as yet unopened losing door, different from your current preference. Then you may switch again, and so on. This continues until there are only two unopened doors left: your current choice and another one. How many times should you switch, and when, if at all?