The GCW leaderboard period is weekly and starts/ends every Thursday at 19:00:00 GMT / 11:00:00 PST / 12:00:00 PDT (same time as the GCW rank update). At the end of the period, the winners are determined, and the list (and everyone's GCW Leaderboard score) is reset to start a new period.
GCW Leaderboard