When I treat with Provent-a-Mite, I've found that if applied correctly in a thin layer, it's dry in less than 30 minutes. I'll usually hold the tub up to the light to make sure it's dry (you can see a difference in the "wet" spray vs the "dry".

If it's been 3hrs and the tub/tank is in a warm area, you should be fine to put the snake back in.