Is there a statute of limitations on unpaid medical bills - after a case has settled?

I had a prior attorney represent me in 2 separate accidents (I was a passenger in one, and not at fault at the other).
Date of 1st accident: October 1997. Last insurance payment made was January 29, 2002.
Date of 2nd accident: March 1999. Last insurance payment made was November 1, 2002.

When the cases settled, the insurance company did not issue enough money to pay off the medical bills.
The Chiropractor sent both of these unpaid balances to a collection agency.

It has been 7 years since any activity (other than interest being added) on these balances.
What is the statute of limitations? Is it 7 years? Am I off the hook for the unpaid balances?

Thank you.