It depends what 'it' is.
Some people find that one session clarifies concerns and provides enough relief to allow them to move on. A follow-up can help to solidify it.
Often people will use between 6 and 12 sessions to work on a problem or set of issues until they feel able to handle it themselves.
Commonly clients come for a year or more to gain deeper perspectives on themselves and their lives and effect more profound change.
One of the questions at an initial meeting will be about what you want to get (and how we are going to go about getting you it).