6 approaches in the last six month period to stay current.
after that, you have six additional months to fly the approaches with a safety pilot in the right seat.
After that youre going to need an IPC with any CFII.
There may be a point further down the line, after a longer period of being uncurrent when you will need to undergo a more thorough checkout, but im not sure.
Search for the FARs relating to this. im sure there online somewhere.