by the hour
1. Per hour; at an hourly rate. Of course he's drawing out this project—he gets paid by the hour!
2. Every hour; once an hour. I feel better knowing that the nurse will check on mom by the hour. Sir, the situation is worsening by the hour—how should we proceed?
by the hour
at each hour; once each hour. It kept growing darker by the hour. I have to take this medicine by the hour.