Well, if it's fully cooked, I don't think you have to cook it. Unless you want to burn it, in which case you should heat at over 9000 degrees for 10 hours.
Actually, you CAN cook "cooked" ham. Most of the hams sold in grocery stores have directions. But generally, do the following: Preheat the oven to 250F. Place the ham into a deep baking dish and cover wither with a tight lid or with foil insuring a very tight seal and cook 12 to 15 minutes PER pound. A rough guide is, and this is approximate, for a half ham 2 hours, a whole ham 3 hours. If you're going to glaze it, have the glaze ready and remove the ham when 10 minutes remain in cooking time. Pour glaze over the ham and back into oven for final 10 minutes.
IF you're talking ham steaks, simply heat through... about 5 minutes.