It is a personal thing. It depends on both parties. There are subs more likely to make friends and there are mistresses more likely to do that. The content of their sessions is important. Sessions high on protocol and role playing are less likely to generate friendships.

Frankly this is not something I expect when I meet a new mistress. However if I meet somebody I like a lot as a person and this can and does happen I want to be friends as much as if we met in different circumstances. Like any other relation it is something which develops over time and organically.