  A large number of AGN have been monitored for nearly 30 years at 22, 37 and
87 GHz in Mets\"ahovi Radio Observatory. These data were combined with lower
frequency 4.8, 8.0 and 14.5 GHz data from the University of Michigan Radio
Astronomy Observatory, higher frequency data at 90 and 230 GHz from SEST, and
supplementary higher frequency data from the literature to study the long-term
variability of a large sample of AGN. Both the characteristics of individual
flares from visual inspection and statistically-determined variability
timescales as a function of frequency and optical class type were determined.
Based on past behaviour, predictions of sources expected to exhibit large
flares in 2008--2009 appropriate for study by GLAST and other instruments are
made. The need for long-term data for properly understanding source behaviour
is emphasised.
