I would suggest you take a quick look at a draft guidance doc from the NDSA "How and When to Check Fixity and see a bit about how frequent organizations are checking fixity in the NDSA storage system peport from a few years back. What comes out in that is that 34% of the organizations from the NDSA that participated in the survey check at fixed intrervals. Which is relativly low, but I imagine will be higher in the future.
Of the 34% that check on fixed intervals htere were a lot of varriance between monthly, quarterly, and annual.
46% check fixity of content on at least a monthly basis.
21% check fixity of content on at least a quarterly basis.
29% check fixity of content on an annual basis.
4% check fixity of content on a tri-annual basis.