I suspect that the duration that OAV is 'active' for post-application is no more than about 5 days. I monitor after treatment and 4-5 days layers. If the colony isn't down to a low daily mite drop by then they get more.
It's worth noting that you can see considerable hive to hive variation in mite levels for colonies in the same apiary - even if they were all coordinately treated in late summer. I think this is due to autumn (fall) brood rearing differences rather than differential efficacy of treatment. Just monitoring one colony in an apiary may be misleading.