Currently, there are two types of leaderboards: Daily and Weekly. To enroll in the daily and weekly leaderboards, all you have to do is play with the G-Loot client open - you will be automatically enrolled!
The Weekly leaderboard collects your stats over a 1-week period, and runs for 168 hours
The Daily leaderboard collects your stats for a 1-day period, running for 24 hours
The starting times for the leaderboards vary depending on game