NFPA 25-2011 14.2.1 Except as discussed in 14.2.1.1 and 14.2.1.4 an inspection of piping and branch line conditions SHALL be conducted every 5 years...
Every 5 years starting the day the system gets put into service.
If you have more than one wet system, NFPA 20 14.2.2 allows the inspection of every other system every five years (i.e half the systems in one shot, the other half 5 years later).