At the beginning of the loan, what you're paying off is almost purely just the interest. You're only making a tiny dent in the principal.

So hypothetically, if 95% of your monthly payment is interest (and only 5% to the principal), making an additional month's worth of payment early would shave nearly 20 months off the loan.

Try looking up some mortgage or loan calculators online and playing with the numbers. Check how much different the monthly payment is with 20 year versus 30 year (might not be as different as you would think). If the calculator allows, try checking how much shorter the loan will be if you keep the monthly payment the same but reduce the principal loan amount by a few payments.

Some people are saying that going from a 30 year to a 50 year mortgage only saves around 5% on monthly payments, probably a bad idea.