30 min blinds?!? Is this live or online? 30 mins is pretty slow online.
Also starting blinds matter. Is it 10/20 to start? Do antes start at a certain level or right away?
For 160 players with 10/20 blinds to start and 30 minute blinds I would guess that you will be 4 hours or more with the blinds at 4 hours only getting up to something like 150/300 or 200/400. Even at 200/400 a break even player maintaining starting stack size of 12,000 still has 30 BBs and is not under that much pressure. Maybe like 5 or 6 hours even to FT.
I don't know of any rule of thumb, sorry. That is 600BBs to start if it starts at 10-20!