I think when it's time to quit, we've come to a place where we feel we can cope well enough on our own or with other support rather than the support of the therapist. It's as simple as that and as complex as that. Or if we start thinking about what else we could be doing in that timeslot other than therapy, that's usually a good sign too. Reading your post I'd let your T know how you are feeling and see what they have to say.