The TL;DR is: you set the standards for what constitutes a good work session (i.e. visual application changes, git commits, etc). You can negotiate a pace / total number of hours with the developer you hire for your project. And you only approve work sessions you are satisfied. You reject work sessions you are not satisfied with and keep costs under control according to your budget and standards. Finally you get ~5 free hours to try this system out — you can use these 5 free hours to gauge the velocity and fidelity of engineer estimates.