For ASV scans, what is meant by quarterly?
The intent of the quarterly scans as prescribed in Requirement 11.2 of the PCI DSS is to have them conducted as close to three months or 90 days apart as possible, so as to minimize the risk and identify vulnerabilities more quickly. For example, if five months elapse between scans (i.e. one done is January and the next done in June), that would not meet the intent of this requirement. In order to meet this requirement, an entity is required to complete their ASV scans, and perform any required remediation, each quarter.