I will usually keep a client in the Stabilization/Endurance phase for about a month. You can usually observe the clients progression in the way they handle the exercises in the workouts you have developed and their ability to handle an increased load. Obviously, some may progress a little quicker or slower than others depending on a number of factors.
After a month, the client may be looking to change things up a bit, so I will move on to strength/endurance. Keep it low key until you feel out how the client adapts. you can then cycle between Stabilization/Endurance and Strength/Endurance depending on the client.