Apparently the "be back in an hour" rule is an unspoken rule. We dove with an operator 2 weeks ago in the keys, they gave us 100's and put us on a dive site that was about 30' deep and told us to be back on the boat with 500 psi. We did a 90 minute dive and all had more than 500 left in our tanks. They were upset with us. If they had said "be back in an hour" we would have.