as i understand it, Bifen is for inside the house, Permethrin is for outside the house;

how often should i apply each?

You can use either of those for indoor or outdoor. Neither has a real long residual. I like to use either of those if I have a known issue for a quick kill for something like fleas or armyworms. Indoor probably 6-8 weeks on either. Outdoor as short as 2 weeks with Bifen and 30 days Perm. I like Suspend Polyzone or Demand CS for outdoor. 90 days on either of those.

After reading this, I bought some Suspend Polyzone from Amazon. 16 oz was $60. I mixed it 1 oz to a gallon of water and applied it around the house perimeter with an electric pump sprayer. It seems to really knock down the gnats that are hatching in the grass. I think Derek knows his stuff!

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Suspend Poly is great. So is Demand. Lots of others that are good. I didn't mention this in my original post but have in the past. I like to rotate the two as pest can build a resisteance to a singular AI and I always add Tekko which is an Insect growth regulator. I could be wrong but if your rotate those and add tekko you're pretty much applying what the "pro's" use on perimeters, but I don't think most rotate much. Imidacloprid drench around trees and other shrubs that flower or have other high pest pressure is great. Don't spray that. Soil drench only. It's a game changer on my Crepe Myrtles and Hibiscus.

Resistance is to the mode of action not the AI. Both of those are pyrethroids as is Bifenthrin and they have the same mode of action. To avoid resistance rotation of modes of action or tank mixing with IGRs and other modes of action can help.