It probably won't make you ill immediately, more likely the texture or flavor would begin to suffer first (hence "best by" rather than "expiration" date). Keeping it stored properly (i.e. not an open bag but something sealed) would likely allow it to last longer.

You should probably not eat 3.5lb of candy within 10 days unless you are trying to make your intestines suffer, but if you choose to binge please update us as to the state of your health so that you may be used as a cautionary tale.