Perth to the East Coast is about 5-6 hours in the air, very similar to a NY-LA flight in the US.

As far as Cairns goes, if you've already seen a lot of other places in Australia and have a good trip planned in NE Queensland then go ahead and stay for 2 weeks. As far as I'm concerned, though, it's not worth spending that much time there if you have the opportunity to see other cities like Sydney, Melbourne, Uluru, Hobart etc for the first time. I lived in Brisbane for 5 months and was not all that impressed, but it might be a fun place to stop for a couple of nights.