I think I wash every two weeks, and the main reason I chose that schedule was because - well, it works for me, and it fit into my overall regime well. The reason I said think is because I use an ayurvedic 'tea rinse' at the one week mark (the weekend I'm not washing my hair), and I feel that helps prevent any issues from the ONE product I use directly on my scalp. I only use a tiny bit of that during a scalp massage, which, in my mind, also helps nip any blockages in the bud.
During the week, I mist my hair daily with a water/humectant/tea tree oil mix, and sometimes I'll do a plain water rinse if my scalp is itchy.
I don't think it has any effect on how my hair grows - in fact I'm going it to encourage both my growth and retention. When I was washing my hair more frequently, I was also manipulating/detangling more frequently, and my hair did NOT approve. As it gets longer, I suspect I'll start to wash it more.