A US FAA license has no expiration date per se; recurrent requirements are detailed in the specific operation type you may be involved in, and may vary between companies (PC, PT, LOFT, etc.). A`"proficiency check" every six months would satisfy most operators regarding currency, and I'm certain that the school where you trained could arrange something generic. Basically another type rating ride (without the FAA paperwork) would probably fill the bill. You won't lose the rating if you don't do the PC - you just couldn't tell a prospective employer that you were current.