It depends what animals you have in the area. Large herbivores like deer and rabbits will continue to eat seedlings for many years. Smaller plant eaters may only be a problem for a few weeks or months. I can't see there being one answer for all situations for this question. Especially when we have no idea where in the world you are or what predators are likely to be attacking the seedlings.
The worst time here is in the first few weeks after transplant when anything that tries to eat the seedlings is likely to pull it out roots and all. I give my transplants protection for a couple of weeks then they are on their own. Losing a few seedlings is no big deal if you have plenty to start with. The less you have the more protection you're likely to give them.