I agree with Merovinge. I would say 9-12 months before starting is probably reasonable. If you're looking into academics, then maybe on the longer side because you have to get letters of recommendations, submit a teaching/diversity statement, etc. Between the verbal offer and a written offer, I've had it take anywhere from 1-4 months, more on the average of 2 months for the offers I've been getting. Then when you get the written offer, having it looked at by a contract lawyer/specialist, taking the time to negotiate, and then finally signing might take another month or two. Also, if you sign at 6 months ahead of time, you MIGHT get credentialed and insurance contracts when you start depending on the speed of the administrative staff, their relationship with insurances, etc.