Depending on the other ingredients, it could take between 30 to 40 minutes to reach that temperature--what you're actually doing is boiling out the liquid so that the sugar gets concentrated. The higher the concentration of sugar, the hotter the mixture gets. Anyway, sometimes it seems like forever to get that hot.

If your recipe calls for real maple syrup, sugar, butter and a little water, try using a tablespoon less water. Don't cut too much of the water or you'll end up with rock candy instead of brittle.

Do you live at a high altitude? If you do, adjust the desired temperature down 2 degrees for every 1,000 feet above sea level so you don't have to let it get that hot(5,000 ft. above sea level, 10 degrees less, so 280 degrees).

Are you sure your thermometer is accurate? Even with a thermometer, I always do the cold water test: at 290 degrees, if you drip some of the liquid into a cup of water, it should form solid threads that are barely pliable.

Try this:

http://desertculinary.blogspot.com/2005/05/chipotle-maple-peanut-brittle.html

If the heat of your liquid sugar is not getting hot enough just increase the burner level very slowly as to not burn the sugar. Also using a heavy, cladded pan will help your syrup maintain its temperature and increase the odds of getting a successful confection.