After about 9 hours of fermentation, use a clean spoon to remove some yogurt (without disturbing the yogurt) to see if it's tart enough and thick enough. If it's not quite ready, program the Instant Pot to ferment for a few more hours. The longer the yogurt incubates (ferments), the tarter it gets. If it's been fermenting for less than 12 hours, it might thicken up with more time. If the yogurt is too tart or sour, you can reduce the fermentation time in your next batch by an hour or two.