Dear dpugliesi,
Fouling factors must be supplied on the data sheet when purchasing an exchanger. These are either supplied from personal experience with equipment in similar service, or via literature values (other peoples experience). Round numbers are used to reflect very little, little, medium, or heavy fouling. If you don't know then you will be conservative, and worst case the exchanger is oversized.
With respect to the original question of cleaning, an exchanger in heavy fouling service will be cleaned every shutdown. It should be sufficiently sized to match the shutdown frequency by specifying fouling factors expected at end of run condition.
best wishes,
sshep