API 570 Section 7.1.2 Newly Installed Piping Systems or Changes in Service, lists 3 methods.
a) A corrosion rate for a piping circuit may be calculated from data collected by the owner/user on piping systems of simular material and service.
b) If data for the same or simular service are not available, a corrosion rate for a piping circuit may be estimated from the owner/user's experience or from published data on piping systems in compairable serice.
c) If the probable corrosion rate cannot be determined by either method above, the initial thickness measurement shall be be made no more than 3 months of service.
So, 90 days is the default as per API 570