I don't know how long it took, but it was certainly longer than I would have the patience for. I clicked on the door once to spawn more opponents (on the premise that a little extra experience would help compensate for a couple mindreads that turned out to be not particularly worth it), but that was enough for me. If you click on the door 30 times in a row, you would end up with enough opponents around you could be taking out groups of them with each swing, so that would speed things up a bit.
