At the beginning it's easy as the teats usually tell you when you need to move up a size (o-3 mos, 6-9 mos etc) as for the bottles it depends. I mostly BF'd my son and he only had the occasional bottle once a week so we were lax about replacing them. But we probably did once every 3-4 months at the beginning, then it depended on the bottle or teats' wear and tear. If they were looking really scratched up and therefore couldnt be sterilized effectively we usually replaced it.
Mind you this was only for the first year and as cautious first time parents. Our fella still prefers his bottles but we've refused to buy any new ones until he gives them up outright..I sense a battle soon to be lost on my part though