If you refer to the USP <62> FAQ online, you get this direction for exactly this type of situation.
Q. With accumulation of the minimal incubation (3 x 18h) for pre-culture, enrichment culture, and sub-culture, it is unavoidable to work at night-hours to perform the validation. How can we avoid these awkward working hours for our personnel?
A. You have to confirm that the test works for the minimum time for routine testing. In fact, should a company find during suitability testing, that the minimum incubation time is not sufficient for a given product but a longer incubation time is needed, prolongation would be a necessary variation of the test.
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