Usually pretty soon
A baby who lives in a clean environment, with clean parents, clean tap water, and a great sewage
system, doesn't need to use sterilized water or bottles after the first 2-3 months.
Sterilizing adds a tiny extra margin of safety, but is a lot of work. Many parents will use bottled water or still boil their water, in case the city water has some microbial spores, although this is rarely a problem.