I don't have any kind of hard or fast limit, but I'll collect ponies until it no longer makes me happy. My wishlist isn't particularly big anymore so I suppose from that perspective there's some sort of limit, though its not like I refuse to buy any ponies off my wishlist if I decide I really want them.
The only thing I'm super particular about is Sundances; I want every single Sundance from all generations (including the ponies I count as Sundance-ish, like Nurse Redheart). Beyond that only time will tell!
I don't have any kind of hard or fast limit, but I'll collect ponies until it no longer makes me happy. My wishlist isn't particularly big anymore so I suppose from that perspective there's some sort of limit, though its not like I refuse to buy any ponies off my wishlist if I decide I really want them.
As someone who has a VERY large Disney's The Little Mermaid memorabilia collection (and yes I know this is about ponies) I think that this is very true for me. Because of how young I was when I started collecting Ariel stuff (I was 9 when the movie came out), I look back at some of it now and go "why did I buy that?" or "why am I keeping that?". So for my MLP collection it is about what makes me happy and not about must collect them all because I have already gone through a phase of that in my life.