I dont "leave therapy"
I take "breaks from therapy"
short version I always enter therapy with a problem that I need help with. my therapist and I work together to solve that problem. sometimes that means addressing more problem areas other times it doesnt.
end result when I find things are going well and I have accomplished what I set out to do, I say see you later sometime.
My therapist and i know that at some point in the future I may give him/her a call and say hey I have this problem, heres what I have tried and was wondering if you have anytime available, maybe together we can figure this out.
then its back therapy for a while until I have accomplished the goals and so forth around the problem and its no longer an issue, and no other problems rising to work on so its time to take a break so see you around some time.
I dont do "goodbyes to therapy" very well, something usually comes up eventually in life anyway. But I have mastered the "its time for a break see you around sometime." approach and it fits in well.