I think I did the math right here...
If you make one extra payment (of principal and interest) per year from the beginning (averaged across all months... so the extra payment is divided by 12 and that amount is added to each month's payment), you'll pay off a 30 year mortgage in about 23 years 9 months.
2 extra payments: 20 years
3 extra payments: 17 years, 4 months
4 extra payments: 15 years, 4 months
5 extra payments: 13 years, 10 months
6 extra payments: 12 years, 7 months
12 extra payments (double your P&I each month): You'll pay off your 30 year mortgage in 8 years, 4 months.
Here are the numbers if you've got a 15 year mortgage:
1 extra payment per year: 13 yr 1 mo
2 extra payments: 11 yr 8 mo
3 extra payments: 10 yr 6 mo
4 extra payments: 9 yr 7 mo
5 extra payments: 8 yr 9 mo
6 extra payments: 8 yr 1 mo
12 extra payments: 5 yr 8 mo
Rob