In Stencil Cleaning process you have many variables that need to be optimized dependent upon the acceptable reliability level of the CCAs. The variables are (at the least):
the type of cleaning solvent used,
the interval between cleaning,
the method of cleaning,
the type of paste being cleaned, and
the size of the apertures being cleaned.
For maximum reliability, you should use automatic underwipe cleaning every 4 to 6 boards using a 2 to 3 component blended solvent that has a much higher solvent strength than IPA. IPA is a poor cleaning solvent by itself, but companies like Petroferm, Florachem, Kyzen, MicroCare, Techspray and others make good ones.
Additionally, high reliability products require that the stencils are removed on a 4 hour frequency, and then washed in an automatic stencil washer or by hand using a hand-held ultrasonic cleaner to blow out the apertures. Your quality control group should be inspecting the solder flow after stenciling to ensure it is meeting your customer’s requirements. If it is not meeting their requirements, then you need to clean more often. It all comes down to the reliability level that you are trying to achieve.