I had the air ducts in the house cleaned (bad allergies) and part of the service was to clean the dryer vent out. I'm thinking: "bah, I clean that out once a year and make sure there is no build up..."
My dryer vents straight up through the roof of my single story house. It's a 14' run straight up. Odd configuration and one I now strongly don't recommend.
Anyway, the guy gets up on the roof with his long brush and starts going to work running it up and down. I live in Vegas and yet it looked like a Minnesota blizzard around my house. Big chunks of lint and lots of lint dust flying all over the place as the technician did his thing. I'd estimate a half-pound of lint total (that's a lot!).
So I dodged a bullet there with that amount of build up. I have added regular dryer vent cleaning by a professional to my house maintenance list.
Hopefully, this will help others avoid a big problem. Get your dryer vents cleaned please!