Body filler not drying?

I'm going to start by sayying this is my first time using body filler. I had a bathtub rust out at work and I do not have the time or money to replace it so I sanded down the area and put some body filler on it. This was 24 hours ago and it is still not hardened yet. The stuff is pretty old but I don't know if that would prevent it from drying or not. Anyways, how long does it normally take to harden for sanding?