Mine have never taken more than a few days...
to the best of my memory, if one or two haven't slit after all of the others have completely emerged, then I never did see a baby slit and come out on its own. I always open those just for my own curiousity. Usually I find a half formed or deformed baby that has some kind of problem and never would hatch. Occasionally I find a live or dead perfect baby that just wasn't able to get out of the egg (maybe he didn't have an egg-tooth - I don't know why). In those cases, I have sometimes had a live baby hatch after I slit it. If it is a healthy baby that is just a few days behind the others, a small slit should allow it to stay in the egg until it is ready to come out. So my policy has been to make a small slit at the top of the last egg (or two) after all of the other babies have completely hatched out.
Please keep in mind that this has been my experience with cornsnakes - other species might hatched out a different rates.