Time in between grows to guarantee no pests?

What is a general time frame to wait in between grows to make sure no pests from the previous grow can make it to the next one? I dealt with spider mites and gnats and am taking the necessary precautions to make sure they wont be here again but what is a good amount of time to wait to make sure there arent any little bugs or eggs floating around i dont know about?

My theory is no plants or soil any stragglers (if there even are any left which is unlikely) can live off of will make sure none transfer over if i wait for their life cycle to die off

Some pests have the ability to go dormant.
My personal answer to your question is 12 hours after you douse every nook and cranny with a heavy pyrethrin based insecticide.

i would bomb the place with insecticide. after a week spray everything with an insecticide such as neem oil or as dbkick says a heavy pyrethrin based insectecide. Then bomb it again with more fog the next week. When your spraying make sure you get it EVERYWHERE!

I am a pest control technician, Bombs are useless. A pyrethrin based insecticide is the way to go. Hit all the cracks and crevices and fan spray the walls, floor and ceiling.
Try to get a residual pyrethrin insecticide.