Welcome.
I've no idea if you know how, but, you'll want to program in some 'checks' to make sure it's not going to 'demote' someone that's in a higher rank.
I could imagine you promoting a friend to a non 'regular' group, they join server for first time, and then when that time check kicks in, they get placed into regular.
Just one thing to watch for (as is true for any time based level system.
Also, if using ULX, make sure the time is HIGH to become regular.
Spending an hour or two on a build server is easy, and can quickly grow your users file to a size that actually slows down the server.
I'd say 24+ hours of play time at the LEAST to become anything besides 'user'.