I stop doing the quests once their objective reads "Talk to Preston". According to Radio Freedom, 'all's quiet in the Commonwealth' after you've saved those two settlements but haven't told Gravy about it yet.
They do actually have some kind of a limit, though I don't know what it is or how it is determined.
I was grinding Minuteman quests for Strong and Curie because normal exploration generated nothing (Curie) or was even detrimental (Strong doesn't like lockpicking) and I didn't want to break character to please them. Eventually Gravy and Radio Freedom stopped giving me quests, and I travelled with someone else for a while before there were more.