So after spending last week in Perth...
I would allow an hour from landing to curbside, and I'd allow 1.5 hours from checkin to boarding. I know that certain airlines issue out express passes for customs, so flying J you'll probably be issued one on arrival, and issued another one on check-in.
Your looking at about half an hour each way from the airport to the CBD via taxi and it cost me $35 to get from Perth CBD to the airport last Thursday night.
Along James St in Northbridge (very near the CBD) there are loads of restaurants, so you'd probably have time to get there and have a decent dinner before heading back to the airport. (Had dinner there last Wednesday night, was very pleasant).