Firstly, it depends on what type you are transitioning from and to? Secondly, it depends on your competence in completing the flight review.
CASA spells out exactly what the requirements are. Complete the form and they will convert your RPC to an RPL with nil actions required.
https://www.casa.gov.au/licences-and-certificates/pilots/pilot-licences/getting-recreational-pilot-licence-rpl#
Stuff required to use the privileges of that RPL. The main cost is in doing the flight review.
I wouldn't put a figure on the number of hours unless I had flown with him/her first to make an assessment.