I believe how long you show on the list depends on when you joined and/or when you last played and your membership level.
Premium Members will be included in the active players list if they have played:
Silver within the last 3 Days
Gold within the last 7 Days
Platinum within the last 14 Days
Diamond within the last 30 Days.
I do think it would be nice to be able to see a full list though (or if there is one, I haven't found it yet).
Hope this helps