Shih tzu....how often do you get you dog groomed?

I have a shih tzu. When I first got her we got her a puppy cut just a trim to get her use to it and they cleaned her ears and did her nails they. Then. About 2 months ago a little longer then that. she got a hair hair cut the whole works. Well she is starting to look a mess again her face has grown up so much. Her eyes are hid in the hair a lot . So how often should I get her groomed is it like every two months or should I just do it when she starts to look a mess. This is the first long haired dog I have ever had. And what kind of cut should she get in the winter its really cold here in michigan in the winter. Just tring to plan ahead.