I follow the directions on the Squadron tube, I wait thirty minutes before sanding. If it's a big chunk then I'll wait half a day because after thirty minutes its still soft.
squadron putty is SH!T i used it on a R32 trying to make a Veilside widebodykit but all it did was sink and sag i tossed the Whole car away. i learned my lesson and begun using 1:1 putty and bondo never had a problem with it yet. another good putty is Dynatron putty & filler works great sands easy too