Personally I would base that off what she is accustomed to getting and tailor it off that.. until you are positive she's learned to control herself, or when you can tell when she has had enough. I just want to share this story, not to scare you, but just to make you aware of what not gaging or knowing the limits can do....
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This was when I really really started to gage his play and exercise. Now.. Titan tells me when enough is enough. He will continue to go go go, he's a WL also, so it's in him to never quit if I ask him to keep going, BUT it's up to me to recognize that. When he starts to not drop his ball, or lays down with it, or doesn't bound right back when he gets it... it's that time to say "let's go inside" Most of the time, if I'm off with my reading, he will look at me like "are you nuts? I was jsut catchign my breath!" but I'm usually dead on and he will bolt into the house and lay down to drink water.
Just learn to read your dog. You know when enough is enough.. you may have to MAKE breaks for her.. especially if she is a go go go type dog.