I have an old Ocean Kayak Sprinter with a permanent bend in the bottom. For years, I threw it in the back of my truck. 8' truck bed plus 2' tailgate is 10'. The Sprinter is 16.
With 6' of boat hanging completely unsupported out in the air, I left it in the truck (black bed liner), sometimes for hours in the Florida sun.
It would dent the hull at the edge of the tailgate, but I could always pop it back out. Over the years, the hull started to get a depression that would not pop out and go away. It finally seems to have "bottomed out" and become a slightly bent boat. I guess at this point, the top of the hull would have to stretch in order for the bottom to bend in any more.
I bought this boat in the mid 1990's out of a rental service where they used to slide it down the concrete ramp. My abuse of it took place over years.
The deformation came gradually. The biggest killer is time, of course. The sooner you can get the boat properly supported or in the water, the better. The next factor is heat. A black pickup truck bed on an August afternoon is just a bad idea, even for a short time.
Keep in mind that as the hull deforms, your straps and tiedowns get loose. Check them frequently when going a long way.
My old blue boat is still fine to paddle. I never noticed any performance change as the hull deformed. You can't even see the bend when the boat is in the water. I finally got a topper for the truck, with a rack on top. I secure a wide board lengthwise on the racks and put the boat on top of that. If I had done this from the beginning, I wouldn't have a bent boat.