Orthotics - how often do you have to change them?

The whole natural running thing makes a lot of sense to me but I am unwilling to give it a go until after my first marathon in April. In fact I find myself going in the opposite direction! My left foot pronates more than the right foot which is much closer to neutral than the left. After seeing a sports physio due to pain in my left leg, which has resulted in a 3 week break from running, I have even advised to see a podiatrist and consider orthotics. I was wondering how long orthotics typically last and how much they cost roughly. Cheers!