Let's ask the other way: at what hashrate loss would a block take 3,10 or 12 hours to solve?
Please, go ahead. Here are the basic formula :
target_hashrate = DIFFICULTY * 7.158278826666667 # target hashrate (MH/s)
actual_hashrate = target_hashrate * (600 / TIME_PER_BLOCK) # hashrate for a block that took TIME_PER_BLOCK seconds to solve (MH/s)