It depends on the variation of video poker (because strategies vary for the different versions), but the rule of thumb is: one royal per 40,000 hands. Thus, if you play 400 hands an hour, you'll hit the royal, on average, every 100 hours. If you're faster and can play 600 hands an hour, you can expect to hit the big jackpot every 67 hours or so.
Remember that this is simply a mathematical expectation -- and your actual results can and will vary greatly.
