My experience of 10-15 is the extra time it takes over the short term carpark option, total. Extra time on the bus in transit is offset by the time it used to take to drive up seven levels of carpark and walk down again.
That would depend of which part of Sydney you are inbound from. If you are coming from the Northern side of the airport, the long term car park is a nightmare, as you have to drive the perimeter road (which in peak hour can take at least 10mins in itself) then get in the car park; then wait for the bus; then drive back along the same perimeter road again as opposed to just pulling up in the short term car park and driving up a few levels. Those inbound from the West and or South it's not too bad as you turn off General Holmes straight into the long term anyway.