With no competition on a given route, and with a good route image (no need of route marketing, just fly the route at least 6x weekly), any plane can have between 85% and 90% and as far as 95% in average. and that's a very good number.
Now, with competition but demand not yet met, you will still have decent 85% LF. the thing about competition is that frequency matters a LOT. Meaning that if you have only a daily flight on that route and your competition has two or more daily, given that both of you operate the same type of airplane, you will end up losing some LF to the other airline.
Also, airplane size matters (on the wrong side), as the smaller airplanes tend to fill their seats more often than bigger ones. For instance, if you have a 737-700 with 136pax and competition has two 68 seat CRJs, he will have better %LF than you.
It could be associated with the frequency thing, as one would need to do two flights on a CRJ to met the capabilities of one 737. But I tend to stay away from that belief, as I've seen several times an 68-seat E-Jet eating a 90-seat CRJ, each one flying 1x daily.
As a side note, route image is important. But you can easily achieve 100 image by just flying the route, the more times you fly it, the sooner you'll get 100 RI.
Hope it helps