Solvent fillers/putties need a light shaping pass after they feel 'hardened' up to take the skin off the top and permit the rest of the solvent to gas out - a night or two in the dehydrator on low will pull it out.
Catalyzed "bondo" type fillers need you to read and follow the instructions. They are most workable and shape-able after they kick and its no longer warm to the touch. This will vary by brand, amount of hardener, etc.
But if you wait too long, the stuff can harden like concrete and be very tough to shape.