I depends how you have the PCs setup, how locked down they are.
You can assign a fixed dhcp lease for each pc based on MAC so that they each always get the same IP address and then create access rules based on IP.
However that's trivial to bypass if the computers are not locked down. If they aren't locked down then you can change the MAC anyway.
I'm not sure if there's a filter based on cumulative access time.
Steve
Edit: Doesn't seem like this is in place yet. See: http://forum.pfsense.org/index.php/topic,36435.msg187789.html#msg187789