I'd recommend getting a cheap stick-on thermometer (called a fermometer) for the outside of your fermenter so you don't have to open it to check temps, and you can monitor it during the most vigorous part of fermentation with just a glance. They have them at homebrew stores, or you can use an aquarium thermometer from Wal-Mart if it goes down to 62-64 degrees or so.
I'd wait a week or two, then check the SG. It's not uncommon to have beers ferment that fast, though- I've had a couple that literally fermented out overnight! It's still a good idea to give it a little time to finish up the last little bit, and then the yeast are still working "cleaning up" after themselves. After the fermentable sugars are gone, the yeast actually digest their own waste products (like diacetyl) before becoming dormant.
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