I;ll be sterilizing till LO is off of formula. The bacteria formed from formula milk is not the same as the bacteria that they lick off of the floor, and because of that it is important to sterilize their bottles while drinking formula. Of course the chances are small, so to each their own. I know people who only sterilize them when they're new and after that just wash them with soap and water (not hot water).
At 3 months we've stopped doing it all the time, i.e. generally, the bottles will still get a sterilise if they've been left too long without cleaning, otherwise just a wash in soapy water.
We like germs in our family though, I know that sounds odd, but I don't want things too clean and sterile.