I do think when/if it becomes time to change therapists, you'll know it in your heart. You'll just feel it. My wife and I had this conversation about her job for years, where she thought maybe she needed to go for a promotion for X, Y, or Z reason, but she was also really happy with the job she was currently doing. I told her that she would know when it was time to move on, and several years later she did. (She also ended up leaving that workplace and industry altogether, but that's a different story!)