This is another part where it’s important to look at the instructions. Some stop leaks are designed to work more or less immediately. Others are designed to work more slowly to help restore older radiators over the next 100 miles or so. It also depends on the damage your radiator has already suffered. Stop leaks are designed to help shore up wear and tear, or fix pinhole leaks caused by that wear. They aren’t really made for large damage or cracks, and in the best case it may take them longer to patch these problems.