It depends how long. If you're doing 2 tatties, then ten minutes seems to about do it. If you're doing four, then you need to give it longer, so I guess I would go by about 5 minutes per tattie. Essentially, if I can't stab them through with a knife after I've finished, they need a bit longer.
Because its a good time saver! I much prefer tatties baked slowly in the oven (with olive oil and salt rubbed into the skin), but time dictates that I don't have the hour and a bit to kill when I get in, then I stick them in the oven to do them first. If I put them in the oven for 10 minutes, then they are perfectly fine, and to be honest, I can't think of anything else I actually use my microwave for. If I have time, I'll stick them in the oven for another 10 minutes, just to finish em off.