You want to be hitting ~6 mins.
@Coffee: Indoor rowing on an ergo can be compared to running on a treadmill. As far a sheer performance goes you can often be better and go harder but it is less fulfilling. The reason is party due to the tedious nature of your surroundings.
That's really good!!! Congratulations! Don't worry, the body works in cycles - sometimes you get close, sometimes you are very far from your PB, but once in a while, you beat it.
I know, a personal best is quite rare when you are an experienced rower. You can improve your fitness but we all have a limit. If there was no limit we would be rowing 500 meters in 1 minute