2 x 6hr cycles, as far apart as possible, one of those cycles to start 1 hr prior to testing.
Be sure you need 12 hrs, seem a bit much? A well balance pool needs 1 turnover per 24 hrs I think ( a few circumstances will require more & sometime you want to err on the safe side so a tad longer; also water temp, debris, sunlight hrs etc all affects this). How long does 1 turnover take w your equipment? The pros here can help you estimate that. Ask away