Great question.
I'd say that if you suffer a horrible set back by a certain product, and you're sure that it's that specific product that caused it--kick it too the curb.
But as far as using products in combination to build a regi give it a solid month. No deviation. You really have to be consistent to learn what's going on with your hair. When you're not consistent then you can't replicate or remember results as well, and you don't really learn much about your hair. Same goes for your skin or for fitness. A month is a long enough period of time for you to contemplate and experience your hair's reaction to products. If you wash once a week though I'd say try it for longer.