At 6 months most of your Labs and Lab crosses have about 90% of their adult size. She isn't going to be a big girl. Switch her to an adult chow if you haven't already. It slows growth, eventually giving you the same size dog, but with sturdier joints. Make the switch over about a week, and stick to the same protean source if you can. Sticking to one protean as long as you can, makes it easier to work around if the dog develops allergies later.
What is she crossed with? Does she look mostly Lab? Many crosses do, but my friends have a Border Collie cross, Lab body except for the BC spots.
Keeping her lean is one of the best things you can do to give her strong joints, and a long, active life. Here is a link that will help,