A pair of sevens should be split if the dealer is showing a 2 through 7.
If the dealer has an 8 or better, take a hit.
Assuming the dealer has a 10 in the hole, and she's showing a 2 through 7, then the dealer might have a total of 12 through 17. All of those hands except the 17 are stiff hands and are likely to bust.
If you split the 7's and assume you catch a ten, you'll have a total of 17. Not only will that beat all those totals except the 17, the dealer has a good chance of busting.
On the other hand, if the dealer has an 18 or better, you're pretty much going to lose most of the time, so there is no point in putting more money in the house's pocket. Just take the hit and hope you get lucky.