There are 8 Time Trials and you will have to beat and the last one is 10 minutes long. Time Trial differs in the sense that your main objective should be collecting pellets and avoiding awakening ghosts if possible. Although eating ghosts isn't entirely a necessity you might want to make the trail behind you smaller if it gets too big, as a large chain will create choke points if it gets too big. Here you'll want to utilize your bombs more if you get in trouble as score is not an issue.
There are 11 total Time Trials, but you only need to beat Time Trials (Short) 1-7 to unlock the 10 minute one. Short Trials 8-10 are not required for any achievements.