This depends on the brand of rowing machine you’re using. Given the Concept2 brand rowing machine is the most common one in American gyms, I’ll assume you’re using a C2.
World and American records for various distances and times are available on theYou can go there and compare yourself to the record holders in your gender and age group.
Now beyond simply tracking your time over a given distance, there are several other metrics you can track on a C2 ergometer, such as power (in Watts or cal/hr), total energy (in calories), or speed (in minutes/500m splits). Most rowers are concerned more with their speed than how much energy they are expending or how much power they are producing, so you’ll most often see “real” rowers using the 500m split display. As such rowers get very well attuned with what kind of splits are impressive and which are not. If I see an otherwise strong/fit looking guy on an erg, and see his splits are above 2:00 min/500m, then either he’s not actually that fit, doesn’t know how to row effectively on the machine, or he’s not trying (warming up, etc.) On the other hand if I see an old lady rowing along at anything close to a 2:00 min/500m split, I’d be very impressed! Switch the display to calories though, and I’d have no idea what’s “good” or not (although I do remember that 200 Watts is about a 2:00 split, and 400 Watts is a very good clip.)
As for your comparison to 90 rpm on a stationary bike: The closest thing on the rowing machine to cadence on a bike would be your stroke rate, in strokes per minute. But before I talk about stroke rate, let’s address the use of cadence on a bike as an indicator of fitness. Cadence is NOT a good indicator of fitness, because it depends on the resistance. I could get on a stationary bike, set the resistance to near zero, and spin along all day at 100rpm no problem. Put the resistance to max, and I can barely turn 40 rpm for a few seconds. So I highly recommend you stop thinking of cadence as a good indicator of your fitness. Instead, look at some sort of energy or power metric, like watts (power which is energy/time) or calories (which is energy, thus calories/hour is a version of power). Expending a high amount of energy in a given amount of time is a good thing to track. As your fitness improves, you’ll be able to expend more calories in a given time, or you’ll be able to expend a given amount of calories in less time. Most exercise machines (cycles, elliptical, rowing, treadmill, etc.) will use a proprietary formula to convert energy into a simulated distance and energy/time into a speed. So cycling at a certain speed for a certain time will be a better indicator of fitness than focusing on your pedal cadence. That said, if you know you can set the resistance exactly the same each time, then maintaining a high (or higher) cadence as your fitness improves is not an unreasonable thing to track, but it’s almost as much an indication of technique/skill as it is fitness.
Likewise, stroke rate on a rowing machine is equally inappropriate as an indication of fitness; actually its even worse than cadence. This is because it’s possible on the rowing machine to row at a very high stroke rate, while doing very little work (energy expenditure). So it’s not uncommon to see people on a rowing machine at a gym who LOOK like they are working really hard (going back and forth very quickly) but upon a glance of the display on their erg’s monitor see that they are actually going very slowly, or producing very little power. This poor rowing technique results from going very fast between strokes (during the “recovery” phase of a stroke, from the finish back to the start of the stroke) when there is no resistance, then not applying much force/effort during the working part of the stroke (called the “drive” which is from the start of the stroke to the finish). “Real” rowers strive for what we call “good ratio.” The term “ratio” in rowing refers to the time spent on the drive, divided by the time spent on the recovery. The drive is when you are pulling the handle away from the flywheel, and the recovery is when you are sliding back toward the flywheel. A ratio of 1:1 or less is desirable. Time on the drive should be less than or equal to time on the recovery.
If you can row with decent technique, and a ratio of 1:1 or less, then your stroke rate should be somewhere in the mid to high 20’s (e.g. 23 to 28 or so) if your resistance is set right. If you find you can row at a rate much higher than 30, I’d say your resistance is too low. That said, another very common error on the C2 rowing machine is setting your resistance too high (e.g. at 7 or above.) the C2 erg is an aerobic exercise machine, it is not optimal for building muscle. For most people with a physique somewhere south of a 1970’s Arnold, a resistance of 7 or less on the rowing machine is appropriate. For example, 220lb 2m tall Olympic champion rowers tend to set their C2 resistance at about 7. Olympic women tend to set it around 4-5. I (male, 2m tall, 220 lb, out of shape ex-rower) set mine in the 5-6 range.
To summarize, and reiterate, tracking fitness on any exercise machine should be done with metrics of energy spent over a period of time. There are many variations for these kinds of metrics, but they all usually boil down to converting energy imparted to the machine over the time it takes to impart that energy. For most machines the easiest version of this to understand will be speed, distance, and time. For rowing machines, Concept2 has a very good, well established, reliable formula for converting mechanical energy imparted to the flywheel into a virtual distance “rowed” in a boat on water. So tracking your time for a set distance, or distance covered for a set time, will be a good indication of your fitness. The Concept2 website also has a surprisingly accurate VO2max estimation calculator, if you’re interested in that sort of thing.