It's up to you. There's no time limit or certain length that you have to feed them. http://www.pregnancy.org/article/how-breastfeeding-benefits-add

The longer the better. The World Health Organization recommends at least two years.

As long as you, and your child, choose. My #1 breastfed for about 11 months; #2 gave up when my wife got a chickenpox abscess at 6 months (it probably made the milk taste bad). In some cultures, children breastfeed until replaced by their successor; in others, children return for a cuddle and a quich snack until four or five.

There are so many opinions out there that it is hard to separate the fact from the fiction. My thinking is that first of all, the outward limit should be parent directed. If the child decides to stop earlier, fine, but you don't want a half grown child pushing you down in a chair and pulling up your shirt, and I've seen that happen. Your child will continue to get some antibodies from you for some time, so nursing will help keep colds and flu down, especially if your child is in day care. But when your child starts eating well from a wide variety of foods, they'll get better nutrition from eating solids. My thinking on it was this: I would let my child wean about like a calf or colt. I'd let her nurse until she began to lose interest, hopefully between 12 and 18 months, and then I'd wean her. Fate intervened when she developed a brain tumor. The first oncologist we saw said NOT to wean her during chemo. I listened to him, and nursed her until she'd had the last MRI after chemo, just shy of her third birthday. The day after the MRI, I told her that she was a big girl now and didn't need "boo" anymore. She patted each one and said goodbye and never asked again. She was ready to let it go.

You can feed the child as long as you want too. Breast milk is very healthy for the child and contains powerful anitoxidants. It can help children with health problems. However, you probably will want to stop when the child is so big it is no longer comfortable to have it suckle.

as long as you feel you want to! but ideally at least a year