It will disengage and not allow it for the rest of the drive. At least that's how it worked 3 years ago when I found this out the hard way. I had to drive manually until the next off-ramp, where I was able to safely exit the interstate. I then got back on the interstate and was able to enable Autopilot again.
I think you will find that it doesn't actually disengage until you provide torque on the steering wheel or you disengage via the brake pedal or AP stalk.