The world health organisation (WHO) recommend at least 2 years. You'll find a lot of us here aim for infant-led weaning.. which can take some beyond 3 years.

12 months isn't really a norm.. I think 12 months has become a bit of a 'thing' because at this age if you're formula feeding it's ok to move onto cows milk at 12 months, so I guess it's flowed on for people to think that breastfeeding can end at 12 months too. But human milk will always be better for them than cows milk, so you might as well keep providing human milk!

If you're happy to keep feeding, then go for it, the benefits for both you and bub are enormous. Enjoy!