Some will say you can stop sterilising bottles after 6 months, some say 12 months, some say don't bother with it at all. Each to their own I say, you have to do what you're comfortable with.

However, the point of sterilising bottles is not that you need to sterilise everything that goes in their mouth. Formula will have all sorts bacteria in it if it's left out long enough. That's why it's suggested to sterilise bottles used for formula, but not for bottles used for EBM.