helicopters are considered "category A" aircraft - circle to land minimums apply accordingly. visibilities can be reduced by 1/2 the cat A min's for helos (but never less than 1/4 mile). Cat A aircraft must remain within 1.3 miles of the runway while circling to land as stated in the AIM -
pick up a set of approach plates and the current FAR's if you don't have them already.....