You should be able to measure 'distance' your gyms machine. A basic workout would be 4 to 6 intervals of 500m with a couple minutes break or going easy between intervals. If your butt isn't kicked at the end, the resistance isn't high enough. Also, form is extremely important. If you haven't already, check out YT for proper rowing technique.
The intervals, distance, and speed aren't set in stone though. You can tweak them a bit to conform the workout to your abilities and goals.
