I think only the last one turned out to be not too popular, but only due to the GM's lack of experience. In general, there should certainly be a break between Mafias, but a short one.
The only thing is, I don't really like the trend of making each subsequent Mafia more complicated. Loads and loads of roles, multiple investigators, two mafias, and so on... But that's my personal opinion.
As for a break, I've said it before: a break is necessary, otherwise it gets stale. At least for me, the old excitement I had in the first and especially the third Mafia is gone (during the second one I slacked off a lot, and then I was hanged pretty quickly :) ).
Bow down and hear our cry,
Our earthly rulers falter,
Our people drift and die,
The walls of gold entomb us,
The swords of scorn divide,
Take not thy thunder from us,
Take away our pride.
G.K. Chesterton: English Hymnal