How long is breastfeeding a dependable natural birth control?

I've heard if you're breast feeding that that alone will be effective in preventing you from getting pregnant again. How long is that true for? The entire time or just the first few months? I plan on breastfeeding for at least a year when I do have children, so would it work that long? Thanks.

http://www.parenting.com/article/Fertili...

Generally speaking doctors say that breastfeeding shouldn't be the only method of birth control, and that you should either use condoms or the Pill, or another method. This is because every woman is different, both in physical make-up and in behavior. If you nurse exclusively for the first year, you probably won't get pregnant. The problem is that you never know when you're going to ovulate for that first time. You could ovulate and become pregnant without ever having started menstruating again. If you are absolutely certain that you do not want more children it would be better to use an additional contraceptive. If you're open to the possibility of becoming pregnant, or want to just leave it up to nature, then breastfeeding exclusively will still probably prevent you from becoming pregnant for a long time. Generally they say that as long as you're feeding the baby around the clock (meaning at night as well as during the day) you will most likely not become pregnant. When your feedings become farther between, and you stop breastfeeding at night, the chance of becoming pregnant increases quite a bit.
I think some women make the mistake of thinking that if they are breastfeeding at all, even if only, say, twice a day, they won't get pregnant, and that's not the case.
So to be on the safe side, if you're sure you want to wait a while for more kids, use an alternative method of birth control.

I breastfed for a year. This is NOT a dependable natural birth control!!

Breastfeeding may delay your period, but you still ovulate!!

Its NEVER a dependable birth control! You can still ovulate while breastfeeding.

You were misinformed. Sorry but breastfeeding isn't birth control. You should get on it ASAP unless you want another kid.