Cooking a ham in a roaster takes 18 to 20 minutes per lb of ham so if you have a 4 lb ham expect to let it roast for 80 minutes.
Some ham is already pre cooked and just basically needs to be reheated to eat so check on the package to see if it's been cooked already as that shortens the time it needs to roast.
Set a timer for 18 minutes per each pound of the ham you're roasting and that will be the time it needs to roast in order for it to be done.