Usually you would want to see a bigger change in interest rates when you already have a fixed rate mortgage. However, you can run the numbers on many online calculators to determine how many years it would take to actually be better off with a re-fi. Don't forget that you will have new closing costs related to the new mortgage. Find out what those are to put into the mix.
In addition, you will need a new appraisal on your property, and the property MUST appraise above the mortgage amount. To get the best rates, you would need to have at least 20% cash equity above the new mortgage amount. With prices falling, you may not get the new appraisal you need to qualify for a new loan as good as the one you already have. Good luck to you.