when i used NAPA's own body filler and hardener cream, it took at least a good 5 to 10 hours for a good cure for sanding and then an overnight cure for hardening...too bad i was at a fundraiser and the thing cured too much, and i had to use a wired power sander, and took 3 hours doing so, then smoothing it out with a finer sander head....
in anycase, u have ample time to work with it, but to make sure, use a small amount, like a good scoop of it or two, and a two inch-ish hardener cream, then mix it fairly well, but not too much or else ur wasting ur time and it gets hardened while you do so.