Put the ham into the baking pan and then put the pan in the oven. Estimate the amount of time the ham will need to cook. A 12-pound ham will take about 18 to 20 minutes per pound to bake, or roughly three and one-half to four hours.At the lowest time estimate, begin checking the internal temperature.When it reaches 160 degrees F, the ham is done. 12-pound ham will take about 18 to 20 minutes per pound to bake, or roughly three and one-half to four hours.