I became friends with the first therapist I ever saw just after I stopped therapy. There is no universal rule about it. The woman was straight and my mother's age so perhaps that helped, but she did have several past clients as friends. We had a hobby in common. Some therapists impose a time frame, some won't do it at all, and some don't seem to care.

It is a bit of a surprise that so many people want to be friends after they quit therapy. I have no urge to be friends with either therapist I see now. I do not think we would like each other particularly or have any thing in common really.