Q: How long (on average) does it take for an adult webmaster to become successful?

A: How long is a piece of string? How high is the sky?

The only concrete answer I can give you to your question is that it doesn't happen overnight. This isn't magic or easy pickins. Most adult webmasters work at this for at least six months before they even see their first sale. Just like anything, success in the adult biz takes time, patience and perseverance.