how do you limit users connection?

one of my sites has a web-redirect portal and freeradius is the way they authenticate. now i want to take it to the next level and use my web-redirect portal to allow 15min connections. Aradial has an Enforcmant switch that you can set those limitations. but how do i do the same with Freeradius? Thanks. -- ____ Sometimes you just glow in the dark...