Maximum exposure time for shooting from helo/prop?

Those of you who shoot from helicopters of propeller planes, what's your rule of thumb about maximum exposure time for given focal lengths?

I won't have time to look and adjust so anything premade would be appreciated

Shooting helicopters... from helicopters includes "Shooting from a Helicopter"

Helicopter Prop Blur bunch of shots

I've only shot from a heli once & it was way back when the 20D was a relatively new camera & my first DSLR. I used a Sigma 18-200 (non OS) and used Sport mode with excellent results. What lenses / body are you thinking of using & under what circumstances.

I'd never consider shooting a fixed wing subject from a rotary wing platform unless the helo was hovering and the fixed wing was just doing fly bys...