How long should I bake the pizza in a microwave oven?

Convection simply moves air around to heat evenly. I don't suggest cooking pizza in a microwave oven at all. If you don't want to use a conventional oven I would suggest trying to cook the pizza on a cast iron skillet instead. The secret to pizza is to cook as hot and quickly as possible, both top and bottom. This is more difficult to achieve in a pan, but if you're using store-bought pizza I would try cooking at a lower temperature with a lid on the pan to create a mini oven. If you have a blowtorch you can cook at a high temp and torch the top. Remember that cast iron heats really unevenly, so in either case leave the pan on the stove for a while to heat to temp evenly before you start the cook.

If all you have is a convection microwave to cook with and the choice is microwave or convection mode then do convection mode as it's closer to baking. Microwaving a pizza will most likely turn it into overcooked rubber, convection gives you a chance. It's a good idea to cut the pizza into manageable chunks that the oven mode can handle, don't cram it in.

I've been in a situation where a microwave was all I have for cooking, I found that I was better off making foods that were microwave friendly rather than trying to cook things that weren't. Try your pizza in convection, if you get an acceptable result it opens up other options to you.