Depend on the temperature that the coolant/heater reaches. Duck tape is for many purposes including quickly fix body panels in racing during a race, but it was mainly created for vent/heater ducts that get warm but not warm enough to burn the chemicals in the tape or cause the tape to ignite.
Also if the section you are trying to fix has a pressure greater than what the tape can withstand at one layer than is not a good idea to use it, since even increasing the layers will not increase it’s resistance due to the fact that is a flexible material and will not maintain a continues junction.
Unless you can layer it continuously and in a vacuum manner what ever pressure one layer can withstand is a good indication weather it will hold to repair the leak. Good luck and do a safe repair job, because I will tell you that if is a radiator hose you are trying to repair unless you are in the middle of the desert and need to get the car to a safe location, you should never use duct tape to repair it.
Also would do at least twenty layers if there was ever the need to do it in an emergency, will only last you about 30 to 60 minutes and probably loose coolant so you would need to check it every 15 to 20 min. I hope this helped.