Difficulty is re-targetted every 16 blocks. Each block is expected to be generated every 2.5 minutes on average. It takes 120 confirmation for a block to mature (confirm), so on average it will take 120 x 2.5 minutes = 300 minutes or 6 hours before a new block gets confirmed assuming the hashrate can catch up with the difficulty re-target.
thanks for the explanation. the only problem I can see with this is that an unscrupulous pool owner can steal the first 6 hours of mining from a lot of people before they can even begin to cash out.
that is always a big danger... but then at least with time, the community figures out who you can trust, and who you cannot