I did a bit of a trip report on this thread:
http://bushwalk.com/forum/viewtopic.php?f=9&t=2526My notes show we went from Lock Ness to the Bunyip entrance in 2 hrs, but this was a slow pace due to checking out exits onto Steadmans Ridge - could probably have done it in 1hr 30min.
We spent an hour exploring up Bunyip and back. So probably 4-5 hrs return from Lock Ness.
Another large group turned back at the amphitheatre which is where you climb the the 3rd fall - out of concern for safety and respect for the local indigenous feelings.
All this assumes you can actually get to Lock Ness by 4WD (or conventional if you are game)
We did this as part of a multi day walk starting from Weetootla Gorge. We were carrying 8 litres of water each as well as gear for 5 days.
Note that I believe the Rangers would discourage people from going up into Bunyip - the local aborigines don't like it, and someone died on the last climb not that long ago.
I only believe this because I had real trouble actually contacting them
Cheers
Brian