It totally depends on your flowering time if you have a 8 week strain it takes 8 weeks to go from clear to cloudy. Sorry to be a smart ass tonite must be the new blue diesel or something, but that is the answer to your question. Now if you mean once they start to go from clear to cloudy then this is going to be strain and envirnmently determined also to a large degree. Once you start seeing cloudy mixed in about two or three weeks of the stated flowering time I would starting checking every few days so you get an idea for future crops.
Good Luck