It's always been 3 hours, and I haven't read of any change (nor have I tested it recently). There has long been an "excursion" fare (I forget how much, but I think $3 at one time) valid if you enter and leave the same station within 3 hours. However, if you exit at a nearby station (say enter at Powell and leave at Montgomery) within 3 hours, you will be charged the one-way fare from Powell to Montgomery. If you overstay the limit I think you have to go to an addfare machine and pay the amount demanded before being able to leave the station.