I think celebrating Mass once a day would be plenty of work for a priest, to preach a good sermon, too.
The devil on my other shoulder says that they should be able to say three Masses a day, given some time to rest inbetween.
When I used to serve mass back in the old Latin days, each priest would say three Masses on All Souls Day, without leaving the altar. In those days the beginning of Mass was “the prayers at the foot of the altar.”
I served for one of these Marathons, too. I don’t remember if we received communion three times. This was NORMAL. This was the PROTOCOL for the day. Certainly, these were turbo-Masses, about 20 minutes each.
Note that when the Church ended that, nobody complained about bringing it back.
Speaking of which, I attended Mass early some morning in my parish which had a lot of commuters that went into Chicago for work. So, the 6 a.m. Mass took about 25 minutes at most, with all the optional stuff out and the shortest possible prayers.