you can't play it just for fun, she will have to do something for it and you have to make sure that the tug of war toy is not always with her as she won't see it as that exciting if she can play with it any time. the length of the game should depend on how well she did whatever it is you asked her to do.
so something ok would be 10 - 15 secs
something really good up to a minute
"In order to really enjoy a dog, one doesn't merely try to train him to be semihuman. The point of it is to open oneself to the possibility of becoming partly a dog." - Edward Hoagland