I was a rower "back in the day". Clubber has said it all already really. I used to set the ergo to "3" on the resistance and most people I rowed with set it there or thereabouts. If you are just training (rather than going for a OB) you should aim to rate between 18 and 22 strokes per minute.
Holding 1:50 (mins for 500m) for 30mins is good going. Just try to do that at rating ~20
Personally, you couldn't pay me to sit on a ergo any more but then I was doing 90 mins on them twice a week for 3 years when I rowed in the Boat Race, which is enough for one lifetime.
Clubber – where did you row?