I understand there is a general rule of 5 mins exercise by the number of months old the dog is. However one of my young bitches loves to swim!

Is there an equivalent with swimming?

The rules for exercising primarily have to do with not putting too much strain on growing bone structure (and especially joints).  I'd say swimming is exempt - it puts no impact pressure whatsoever on bone - which is why it is so beneficial to dogs recovering from any type of bone surgery or arthritis. 

at what age do you stop the 5 minute rule, please

I'd say round about a year for all but giant breeds - after all, at 12 months you'd be giving walks of an hour (if you apply the 5-minute rule as a total per walk, not per day), which is as much as many people give.

Thank you - I knew it was until they had matured but someone in another post said their Golden Retriever didn't "grow up" until it was 6 years old so had visions of walking him for 6 hours a day by that time as I hate to short change him with his walking and always make sure he gets his full allowance of minutes :)

I am sure he would be more than happy to have 6 hour walks

Ah, I think people were referring to mental 'growing up', not physical. ;) I know my breed generally doesn't mentally mature until about 7 years of age, although they're never totally 'grown up' - which is why we love them! :D

No can't possibly refer to mental 'growing up' for a Golden, 6 is far too young :D.