I am paying $50 extra per month on mine (all I can afford until the student loans are gone) - I set up a spreadsheet based on that, and my mortgage will be paid off 7 years early - not great, but before I retire (which is my goal). However, every time I get a raise, I pay a few extra dollars to each of my bills - I'm concentrating on the student loans (highest interest rate - 7%) and the second mortgage (cheaper than PMI); when they're paid off, the payments for each is going to the mortgage, which will then be paid off in 2022, instead of 2033 as originally due - but I'll accelerate each of those as often as I can, so I'm aiming for sooner.
If you want to set up your own spreadsheet, you need an equation that looks something like this: =SUM((D3+(D3*(annual interest rate/12))-payment))
In this equation, D3 (column 3, line 3) refers to the location of the original balance of the loan, or the balance in the previous month; the annual interest rate should be replaced by your annual interest rate, as a decimal (so 5.5% = 0.055), the 12 is 12 months per year (you can figure it out daily, but the difference is pretty minimal - and this will give you a good estimate; mine is usually within a dollar or two when I check it); the payment should be your actual mortgage payment - don't include insurance, taxes, or anything else that is added to your mortgage payment, or you won't get a good prediction. Once you have the equation set up, you can copy it into cells in the same column as many times as you need to make your projection. I put the date down the left-most column.
You can then copy the equation to the next column and change the amount of the payment to see how much difference paying extra will make in your payoff date - and remember that anything extra you pay to principal is money you will never pay interest on again - just make sure your mortgage company is applying it as extra payment to principal and not overpayment toward the future.
You can use the same basic equation to determine the payoff date of any debt, by adjusting the interest, payment, and starting balance.