In warmer weather and <= 45mph, it could be as much as half of the drive, depending on terrain.
Higher speeds, cold temps and or a head wind, a lot less.
I think the highest I've seen mine in electric, was 65ish and for about a mile but conditions have to be just right for that. IOW, down hill, no head winds.
Around a third of the 12K on mine, are electric only miles.
I've been trying to keep track of mine to figure out the logic. The highest I've had it go electric just shy of 70 so far but that was downhill. One thing I noticed is that on flat ground, the electric motor isn't able to accelerate the truck much, if at all, above 50 or so. If I go into electric mode above that it will either maintain speed or slowly lose speed when I'm holding the blue bar at the max level on the EV coach. I haven't kept track of max distance but I can't imagine I've gone much more than a mile before needing the engine to kick in (people drive very erratically here).
So far I've been able to hit ~40% electric miles on most of my drives where traffic wasn't too bad. It's been neat to watch and try to figure out the programming to try and stretch it as much as I can.