A while back I was looking at soaking a mosquito net in permethrin to keep no-see-ums away. I contacted someone from Sawyer and asked them a few questions.
First of all, the soaking treatment lasts longer than the spray treatment. Personally I would go that route if possible (easier for a mosquito net than clothing, however).
But I also wanted to ask why their treatments didn’t last nearly as long as one I found from the UK (Lifesystems or in Military versions. He told me that basically the US government would only allow them to sell a certain amount or concentration of permethrin. Of course, he would never suggest such a thing, but I understand that if you put the same amount of permethrin in you’ll get the same results as the longer treatments.
He said that it was important to seal the net in a dark plastic bag in between uses. It’s important to reduce light (I imagine UV) exposure and air flow. I asked if it would last longer if I did that and only used the net once a month. He thought the treatment would last much longer than the advertised six weeks.