How many minutes between 115 F and 120 F for a ham?
Probably another 10-20 minutes. The interior should be heating somewhat faster now than it did initially, since it took a while for the heat to propagate all the way in. And assuming it was refrigerated at 40F, it's averaged about 10F increase every 20 minutes. (Your 15-20 minutes outside the refrigerator isn't going to change that much.)

It's also really not that sensitive, so it doesn't matter that much if you over shoot. So I'd check after 10 or 15 minutes, and if it's not all the way there yet, you'll at least have a good idea how fast it's heating.