It depends a bit. If you think that say 2 people rebuy, thats an increase from 1600 to 2000. Now the "20 BB rule" says that it will end when the BB is around 2000/20=100.
So what does this mean in practice? Well, it means that the tourneys will drag on to 80/120 more often then they will in a freezout. But still, even then there will be 2000/120=16.7 BBs in play, so it should end fairly quickly after that.
If everyone rebuys? Then the chip count doubles from 1600 to 3200. 3200/20=160, so it will likely last another 2 levels. But at 13 minutes per level, that's just 26 minutes, perhaps not the end of the world if it ever was to happen?
Once again, it's impossible to know when it ends. My experience is that for 2h turbos the "20 BB rule" is more often breached than it is with longer tournaments.
Hopefully you will have understood the reasoning now? How to calculate the last level based on chipcount? If not, don't hesitate to ask and I'll try to explain further, but the steps in general are:
1. Estimate the chip count
2. Divide it by 20 to get the estimated last level
3. Find a structure you like
4. Set the level length by dividing the playing time (minus breaks) with how many levels there are to the estimated last level.