You can breastfeed a child for as long as you want, your supply will go once you stop feeding.
When you say any extra what are you meaning? If you mean extras on top on breast milk then personally no as breastmilk is all my LO needs at the moment and I am around to give it to her. Some people will combi-feed - use formula when they are unable to feed LO themselves, others express. My LO won't have anything apart from breastmilk until she's being weaned onto food.