That depends on the internal condition of your cooling system and the type and quality of stop leak used. Reportedly General Motors used the Bars stop leak type pellets in every new radiator for years, this was their way of insuring that they would hold, as told by a retired engineer who worked for GM for over 35 years and he said that was the normal way of sealing new radiators. Stop leaks can plug up the system where you do not want it to as well as if you ever have the system serviced it may leak again. Certain parts of the system seal better than others which depends on the material such as plastic, rubber, iron, aluminum, ETC... and the pressure and temperature. Water pump seals and oil coolers rarely are helped by stop leak.