How long does it take to warm a ham in the oven?
Since it's already fully-cooked, you don't need to be all that precise with time or temperature.  The most important thing is to avoid drying it out, so use a roasting pan and add a little water to the bottom (not more than 1 cm) and cover it while it's reheating.

It's best if you have a meat thermometer; toss it in at 300° F / 150° C (or up to 350° F / 175° C if you're in a hurry) and aim for a temperature of about 140° F / 60° C.  If you don't have a thermometer, this about.com page suggests 16-20 minutes per pound; it also suggests heating to 135° F which is also fine.  But if you're not going by a thermometer then you'd better make sure that it's completely thawed first, all the way to the interior, lest you end up with a semi-frozen dinner.

If you also plan to glaze it then apply the glaze after it's been heated and raise the temperature to 400° F (205° C) as the above link suggests.