I think too much direct heat is using any type of heating appliance more than once a week.
To me, I think flat irons/curling irons are more damaging than blow dryers/hooded dryers.
I was on the no heat challenge for 4 months and it's definitely helped my hair thicken up, but I just couldn't deal with my hair looking the way it did from airdrying. I would continue to do airdried rollersets but I have too much new growth to rollerset my hair anymore. So now, I'm starting back with blowdrying and rolling my hair when its dry with oils.