When they give you an estimated wait time, it's just their best guess. If enough parties linger after dinner, order dessert, etc., then that throws things off.

Also, not only do I agree with p d that you should have spoken with a manager if it bothered you that much, I don't think that a one-time incident warrants an email to company headquarters. If it happened regularly and speaking with a manager didn't resolve things, then fine, email company headquarters and let them know it's a problem.