I waited a week one time for the seed shell to fall off and it never did so I pulled it off. Interestingly, the plant ended up being the smallest out of the batch. I'd say the quicker you get the shell off the less likely it is to be stunted. Of course, you risk serious injury or even death to the seedling if you don't perform the operation correctly. If you're too worried about messing it up, just wait a few days. There's a strong possibility it'll fall off on its own.