Best thing to do is see what kind of rate you can get for 15 years, or whatever period you're looking at, then run the numbers to see how long it would take to recoup the refinance closing fees. You'll hear people say, "if you can't drop your interest by at least 2%, don't do it". That's just something they've heard somewhere. Run the math and see for yourself to make sure. I will say though, as low as your payment is already, if the interest rate isn't that much if a difference then just double your payment. Basically same thing as changing to a 15 year from a 30 year. Just make sure, if you do double your payment, to mark the extra amount as "towards principal". If you don't, they may just record it as making early payments and it won't lower the amount if money you are paying interest on.
