The answer to your question depends on several factors.
1. What is your definition of a "decent" witch? Are you referring to a witch who is powerful and able to conjure quickly? Or are you referring to a witch who is Good and "decent" (humane)?
2. Which witch path are you following?
3. How good are you at focusing and consistently practicing daily rituals?
4. How good are you at decerning the subtle differences in the ways of the world?
5. Are you patient?
How long or short, easy or difficult any path might be, is really up to each person and/or the coven/group/sect/path they feel most called to follow.