It's 7 yrs per hour, as they calculated - however, counting from descent to arrival back at the Endurance (remember Endurance stayed in safe distance, not in the immediate orbit. Time distortion started for them soon after leaving). So: descent, landing, wave hitting, engines flooded. CASE estimates it will take 45 mins up to an hour to drain. When the next wave is approaching, the engines still need 2 minutes to drain, so around 40-60 mins have passed. Cooper sparks them and they get back. Landing, time on planet and getting back to their ship not in immediate orbit - this all took them a bit more than 3 hours in total. 

Brand says "I don't know. Decades" as a desperate short reply immediately after CASEs estimation, in that moment they all don't know for sure how many hours it will take them to get back to the Endurance (will the Engines go back online immediately when drained or are there any other drawbacks ahead?). All they know is they can't start right away, it will be app. an hour at least before liftoff and the entire trip therefore longer than estimated.