I just defrosted about one pound of hamburger in my fridge, and it took a little more than one full day. The amount of hamburger, packaging, and setting of your fridege will determine the answer for you.
Frozen ground beef patties should defrost in no longer than 24 hours. You do not want them to defrost longer and chance bacteria. The patties should be cooked within the twenty four hour time frame. Do not defrost outside of the refrigerator for any length of time. Safety is a concern for defrosting meat.