is there a ruling on how long (if at all) a demon/devil who dies on the Material Plane is banished to the Abyss/Nine Hells?
Page 67 of the MM ensures they’re sent back, but not if they are barred reentry. No hard word on banishment/lockout as far as I'm aware. But they can't generally re-enter on their own. They need a portal which is incredibly rare, or have to be summoned by mortals. #WotCStaff

— Dan Dillon (@Dan_Dillon_1) March 5, 2019

For anyone perusing at a later date, this is on page 66 under “Dark Dealers and Soul Mongers.” I was looking at “Eternal Evil” under demons. 🙂

— Dan Dillon (@Dan_Dillon_1) March 5, 2019

Indeed, I’m focusing more on the devil aspect ATM, so I remembered where that one was xD Yep looks like the same deal there. Under “The Infernal Hierarchy” it talks about instantly reforming and that’s all.

The main issue is there’s no easy way from the outer planes to the Material Plane. #WotCStaff

— Dan Dillon (@Dan_Dillon_1) March 5, 2019

Agreed. Just wasn’t sure if there was a time-lock for reentry anymore! I feel the idea of being poofed back into what is essentially PRISON should be enough to give troublesome adventurers a few weeks’ respite at the very least.
But a devil with a loyal summoner? Woof. Oh my yes. But that also means they likely need to trust someone with their true name.

— Dan Dillon (@Dan_Dillon_1) March 5, 2019