Simply, it depends on how long you want a partner helping you create good things in your life!
If you have a very specific short-term goal you’d like to achieve, you may work with your coach only for a few months. (It’s generally not recommended to work with a coach less than three months…It’s often hard to permanently integrate change in a shorter period of time. You need some real-life applying your new knowledge time!)
Many people create some wonderful results with their coach, then just keep using them! A relationship with a coach that “gets you”, knows how to motivate you, and is familiar with your common “pitfalls” can be absolutely invaluable.
Those that keep their coaches on for years, tend to go on to accomplish significant change or achievements, such as serious career advancement, major career change, or writing a book, for example.
Perhaps the most significant achievement for people that keep their coach on long term, is their level of personal fulfillment, and amount of positive contribution they make to the world.