For me, the cutoff is about 8 hours, unless I need 4WD, in which case I often just suck it up and drive regardless of the distance.
For an 8-hour driving trip, it takes me 1 hour to drive to the airport (your situation will vary). You have to be there at a very minimum of 1 hour before boarding (your situation will vary). Boarding/unboarding is about 45 minutes. A flight that long is about 1 hour. And then the forced baggage claim since flights that short often don't allow carry on adds 30 minutes. So, flying a trip that length takes a minimum of 4.25 hours, and that is if there are no delays, or rental car lines. I'll have to cough up $200+ to save 3.75 hours. I do it, but grudgingly.
Any shorter that that and it is never worth it.
*Go online, rent 4WD truck, arrive at destination, rental person hands me keys to an Infinity station wagon (no lie, this happened). "That's not a 4WD truck like I reserved." "It's all wheel drive!" "It's isn't a 4WD truck like I reserved and you promised me." "It's all we have, take it or walk." Shredded a tire within 20 minutes of going off pavement and it wasn't even a bad road.
Exactly. I often go to obscure places with roads that have ruts large enough to swallow small cars, no pavement for 50+ miles, and certainly no cell phone service. So, when I rent, I want 4WD and high clearance.
I've rented a 4WD Jeep. When I arrived, they said that I can take any vehicle that I want from a row. No Jeeps at all in that row. So, I take the closest thing in that row for high-clearance and 4WD, get to the exit and they say that'll be another $100/day. So, I return that, and drive the highest-clearance 2WD that I can find (usually some crappy vehicle that can barely do what I need).