If he is up to full fitness then once a fortnight would be fine just remember if he has any periods of discomfort or is reluctent to excercice then u can always increase it
Just a thought.... we have a dog that attends every week (it was every day a yr ago) his insurance ran out and his vet had a word with the insurance company explaining that if it wasnt for hydro he would be claiming for more pain killers he got an extra 20 sesions paid for.