I'm just beginning with this, so am on the novice level. I got some pointers from the Concept II site, and have set the slider to position 4. I wasn't spending long enough warming up, for a start, and then I tended to keep the same pace throughout, around 30 spm for 10 minutes. What I now do is 5 minutes warm up at around 26 strokes per minute, then hammer it for a couple of minutes - hitting 33 or 34 spm, then back down to an easy pace. I do three of these 2 minute bursts (definitely gets your heart going), with a couple of minutes of easy rowing to recover between each one. Then 5 minutes warm down again at around 26 or 27 spm.
So yesterday I did 3.5km, which took 17.5 minutes.