You don't have to marinate tofu but if you do want to infuse it with flavor, I recommend at least half an hour. You can marinate firm and extra-firm tofu for up to 3 days, although that only applies if you aren't using much salt or acid in the marinade mixture. Soft or silken tofu shouldn't be marinated for more than 24 hours else the proteins will begin to disintegrate and it will fall apart.