This depends on how fast the wheatgrass is growing which is affected by temperature. The warmer it is, the faster it will grow and the growing period will be short. If it is kept in a cooler area, the growth can be slowed so the plants can be used for longer. Some wheatgrass juice bars will put the trays in coolers for a few hours at a time to slow the growth so they can be used for a longer period. If it's warm and the growth can't be slowed, it's best to harvest the whole batch within a couple days.