
It depends how fit she is really :)
Is she an ex reputable breeding or puppy farm girl ? All depends on how she is healthwise as well as some Labs even at that age can start to suffer from athritis so the type of excercise is then important :)
If she is a well, fit and good weighted girl she could have anything from one to two hours mixed with of and on lead excercise.
If she is over weight or not 100% healthwise then about half to one hour with perhaps 50% off and on lead.
Most labs if ok are quite lively and able to tolerate reasonable excercise. Min my old at that age used to beat two border collies both younger in fetching a ball

I would wait to see how she is when you get her and take it easy over the frst few days and assess how she is then adjust the amount once you know how much she can tollerate :)
Lucy
xx