I've read a number of articles and had conflicting advice regarding how much exercise a labrador should have, and the possible joint problems etc that could occur.  Can anyone shed some light or help as i have a six month old black labrador bitch and would like to know how long i.e. time and distance should she be exercised. 

The usual rule of thumb is for 5 minutes exercise per month of the dog's age. So a six-month lab would have walks of 6x5 minutes = 30 minutes.
:)

actually im interested, when do you stop the 5 min per month rule? at 12 months when the walk is about an hour? and then what do you do if you want to walk further just carry on building the dogs stamina and fitness up?

The 5 minute rule is a great easy to remember, easy to use rule and one I always quote. So saying, it will always be a compromise. The development of a puppy is not a straight line. A puppy develops at varying speed during the first few months of its life. Possibly for the first few times out at 12 weeks old 15 minutes is too much. Around the middle area of development, say around 6 months, 30 minutes is ideal. Moving on from there, a dog such as a Labrador is approaching full size at around a year old and as such could go a little further than the 60 minutes. Just to add to the complications, I have never had a Labrador fully grown until around 2 years old so in theory the bones are still growing until then. Some lines of Labs seem to be finished growing by a year old so their bone growth will be done by then!

From all that you can see the complications involved in answering this question and really there is no one answer which would suit all dogs! I think I'll stick to stating the 5 minute rule, It's probably the best advice we can give.

Best wishes, John

Would that be once or more than once a day?

That would be twice a day at 6 months. Younger than that can do three times a day, because then walks are not for exercise purposes, but for socialisation and training.
:)

I would have no problem with a couple of walks during the day provided there is plenty of time for rest between. The 5 minutes a month times are also guidelines, not cast in stone. A few minutes longer occasionally will do no harm. (Providing it is only a few minutes :) ) I would just balance it out with a little shorter walk next time. The aim is a steadily increasing amount of work as the puppy grows.

Best wishes, John