Auto-Assign new usergroup after X time

Hello Xenforo Community!

I am looking for a way to auto-assign users to a new group after they can been in a certain group for 30 days.

For example, SirSloth has joined my guild and has been promoted to Guild Recruit. In 30 days, if SirSloth is still in the Guild Recruit usergroup, he automatically gets promoted to the Guild Member usergroup.

How hard is this? Is it possible? How much will it cost me?

Thanks in advance.