Abrexa, as Lazer suggests it's hard to see a clear pattern with FAST's update cycles.
Obviously they are not (yet?) updating the complete 2,1 Bill pages in one go.
Statements from FAST indicate there's a popular section of the db, which gets updated more frequently, ca every 9 days, than the rest.
See this thread:
FAST / ATW update cycles [webmasterworld.com]