I have repeated about 6 or 7 sp's about 4, 5 times each. I have seen Tia about ten times. Then there was one where I left ten minutes in.
It's just that sometimes you feel that she has offered as much she could offer, and you move on. Regarding familiarity, I think it varies. Being more familiar can bring boredom or it can also make an SP more comfortable so she opens up more with repeat. But all good things do come to an end.
Another factor is what is out there. If I lived in a small town that had fewer sp's, then I would have repeated a lot more. There are sp's out there that I saw only once not because they were not good but simply because there are more options out there. Living in greater Van is that there is so much out there for same price and being a guy, I like that.
Just like anything else, as long as things are fresh and exciting, you repeat.