Heating meatballs in pasta sauce
20 or so minutes cooking the meatballs in the sauce has always done the job for me. You want to cook them sufficiently long so that they absorb the moisture and flavour of the sauce, but not too long such that they lose their firm texture. The reverse process of the meat flavouring the sauce is also an important one.

As a side note, it is more advisable to simmer the sauce, rather than boil it, or it will likely become too thick and may even burn.

In general, the combination of initially cooking the meatballs in the oven, and then simmering in the pan is the right way to do things. This ensures that the meatballs are sufficiently cooked, do not fall apart, and have the right firmness. Worth noting is that you should pad off the grease on the meatballs before putting them in the sauce.