resistance level is very machine-dependent - level 10 on a badly maintained machine can be easier than level 2 on a new one because the fan casing fills up with dust and the fan doesn't slow down as much between strokes. If it's your machine, it's worth hoovering the cage out every year or so.
to check what resistance you're really working at, go into 'more options', then 'view drag factor' and take a couple of strokes until you get a number, it should be somewhere between 80 and 200.
Most club rowers will go for about 100-110 (women) and 115-125 (men). Go lower if you're less experienced, it's slightly more technically difficult* but it'll be **much** less likely to cause injuries on longer sessions.
* - you need to make sure you're just using your legs in the first part of the stroke. If you try and use your body or arms too early then it'll make it more difficult to generate enough power.