NCLEX prep--how much is needed?

Help! Any suggestions welcome.
I am an experienced RN who has been working in Australia for 20 years and for the last 10 years in acute psyche. I'm now thinking of having an extended working holiday in the USA.

I contacted the state nursing board I qualified in before I moved overseas and they said I need to retake my boards to again work in the USA. Took them last in 1980 with no problem but it's been awhile and as some may know psyche is a bit of a different area.

So my question is how much prep do I really need to do? As my only objective is a pass and I plan to look for work in psyche.

Thank you!

hawaiitohobart