The best way to determine if the battery needs an equalizing
charge is to check the specific gravity readings for each cell. If
there is more than 0.020 specific gravity unit variation between
any two cells, the battery should be equalized. A good rule of
thumb is to equalize the battery once each week.(From Dekas Battery service pamplet/brochure)

This depends on the weekly usage of the truck if is only used once i a great while will be longer compared to a truck that gets used 8+ hours daily