If an Australian licence I believe they remain valid for life but require you to have a current medical and flight review/proficiency check within the last 2 years. You will also need to apply for an AVID (aviation ID).
If an Oz licence, do you recall your ARN? If so, you can contact CASA and they will be able to tell you what records they have of your licences. try:
[email protected] (or go to web page
https://www.casa.gov.au/licences-and...sked-questions)
You will need to be issued with a new Part 61 paper licence at some point - you can apply yourself or can do it as part of the flight review/proficiency check paperwork. It is a little fiddly if you have lots of log book endorsements for things - they will all need to be certified and copies sent to CASA to get them entered onto the system with them - probably best to sit down with someone at a school or club who can talk you through it and then you can go away and get the ball rolling with medical and AVID application prior to doing some remedial training and then a review/proficiency check.
As far as amount of dual prior to the check?... really depends - you may pick the hand skills and procedures back up really quickly or it may take a little while. I would guess somewhere between 5-10 hours dual? Depends also on aeroplanes you flew etc. A good instructor should be able to work out a plan to get you back to speed and give a rough indication of time after doing a bit of flying with you to see where you are at.
I can recommend someone to talk to about the process if you want to - PM me.