QC must be performed in accordance with all applicable local, state, and/or federal regulations. Your laboratory's QC procedures will be written based on these regulations and the accreditation requirements that apply.
At a minimum, the intended reactivity of the Gram stain must be verified with every new batch of stain and at least weekly against known gram-positive and gram-negative control organisms. However, individual laboratories may have more stringent requirements. No matter what the required frequency for QC testing, if you prepare and read Gram stains infrequently it would be helpful to stain QC slides at the same time as a patient Gram stain smear.