I read the newsletter about using garlic powder to deter mosquitos, and that this technique should last around 20 days, wet or dry, before having to cast out some more. I'm finding that it works, but it only last at most, a couple of days before the mosquitos are back, swarming on my covered concrete porch. I've put it in the yard, on the concrete, everywhere. Why isn't it lasting as long as it's supposed to?

You should be using a granulated garlic. What are you buying?

It's a container from Costco Wholesale, Kirkland brand "Granulated California Garlic" made from 100% california grown fresh garlic that has been dehydrated and milled to size. Is that wrong?

I think you should resort to another method to repel mosquitoes. You can always purchase a mosquito repellent from a home improvement store. Or, try these do-it-yourself methods:
1. Add a sprig of rosemary to the coals of the grill
2. Use Citronella Candles.
The scent these products releases can repel mosquitoes.
Hope this helps!