I’m guessing it’s a mercury compound rather than metallic mercury, so it might have different properties.
I’ve done a little research on these bulbs, and the amount of mercury in them is much smaller than I realized. Typically, a bulb will contain about 4 mg of mercury. As the bulb is used, much of the mercury bonds with the inside of the bulb (I haven’t been able to find out what compound is formed).
So, you could evaporate the mercury in a reasonable amount of time provided you circulate air over the mercury. The assumption seems to be that opening the windows will create a draft over the spilled mercury. If the mercury was in a recessed area, this might not work. It would seem to be more effective to set up a fan to gently blow air over the area where the mercury is spilled, keeping the windows open.
It would take about ten times as long to evaporate the mercury from a new bulb as it would the mercury from a bulb which has been used for a long time. I wonder whether the professor’s 20 minutes is for a new or used bulb?