Boiling meatballs, how long?
Getting meatballs done is a matter of raising them to the correct internal temperature. 30 minutes seems like a long time even for large meatballs, but it depends on many factors. The best way is to pull the biggest one out and test it, first see if it's firm or squishy - firm means it's getting close to done. If it's firm stick an instant read thermometer into the very center of it, if the temperature is at or above 160F (70C) then they are all done. If you test a medium size one the larger ones may need a little longer. 

Incidentally, boiling meatballs is not the only way to do it, and my least favorite because some of the flavor is lost to the water. You can bake them, fry them, or cook them in a sauce as well. Cooking them in a sauce imparts flavor to the sauce, you'll get the fat from the meat too, though.