Starting a business from scratch, for instance, could take anywhere from 6-12 months.
Developing your self-confidence and public speaking skills to deliver a presentation at work or university could take as little as 1-2 months.
Typically, most clients will take up 3 months of coaching initially. Depending on where they are at the end of those 3 months will determine whether they feel they can go it alone from that point onwards, or whether they would like to add extra sessions.
I provide my coaching services month to month.
Within each month, the client will get 2 coaching sessions (one every 2 weeks), with 24/7 support between each session (within reason, although I have been known to have 2am coaching calls with clients in the past!).