what kind of mushrooms? button? whole or sliced?
I'm not sure I'd steam mushrooms, but then again your not asking my opinion on the cooking technique, so I'll answer your question the best I'm able with information you supply me.
in the mean time, doness with a mushroom is relative. since you can eat a mushroom raw, doness becomes a matter of application. For instance if you wanted to steam a button mushroom whole for a salad, simply because you didn't want to eat it completly raw. I'd steam it for just a minute or so, so the mushroom starts sweating, is softer but still has a firm body for texture in the salad.