If you purchase the "Mod Trial" you will be set off as an moderator after being mentored. The rank stays for a period of 30 days. Means that after 30 days of work, you will be needing to apply for a new rank (Admin or extention to your rank).
Admin does not have this sytem at the moment, Will probably be added that after 3-4 months you will be needing to extend.
Easily said, Moderator: 30 days period, then you apply for further rank.
Admin: No limit. If a limit is set of a time period it will be months!
Hope this answered your question! If you still feel unsure over what i wrote to you, check this post out!