My boxer pup is nearly 18 weeks old. I have been taking her to the park (she has been twice this week) for about 15 minutes each time. Is this enough or too little.. Was advised not to over exercise because it can cause hip damage when she is older. Would appreciate any advice on this. I am planning on 2 park trips a week and playing in garden and a few lead walks a day.

I would think that's fine. Most of her outings will be on the lead for training and socialisation purposes, but some off-lead free play is vital too. (All work and no play....)
Good luck. :)