If you're doing wok cooking, you can get to 900 degrees, although most of us who did not have the gas company adjust the stove for optimal Chinese cooking don't get quite that high. 600 F isn't crazy in wok cooking on a stovetop, though, and Teflon is not guaranteed to retain its properties above 400F. [1,2] Here's a PSA: Teflon-coated woks suck anyway!

Remember that Teflon and related products aren't only for stovetop use. There are many non-stick baking sheets and muffin pans as well, and it's not that unusual to cook muffins, scones, cakes, or pizza (esp pizza [3]) at temperatures above 400. Moreover, using the broiler can quickly get you to a high temperature.

Interestingly, there are a lot of documented cases of bird death in conjunction with Teflon usage [4]. Birds are very sensitive -- hence "canary in coal mine" -- but it is interesting to see that bird death can happen during normal use of Teflon and related products, certainly at temperatures below 400F.

[1,2] http://www.seriouseats.com/2012/06/the-food-lab-for-the-best..., http://www.boedeker.com/ptfe_p.htm [3] http://www.thekitchn.com/recipe-homemade-thin-crust-pizza-re... [4] http://www.ewg.org/research/canaries-kitchen/teflon-kills-bi...