It depends on the type of tournament you’re taking part in.
If you’re playing in a real money tournament, you can play whenever you like throughout the duration of the tournament. You will find this information in the tournament details.
If you’re playing in a play money tournament, a game lasts from 10 to 15 minutes, starting when you begin the tournament by clicking “Play now”. After this time, the game will be interrupted, even if you still have credit. The tournament will also end if you have no more credit but there is time left.