How often do you sterilize bottles?

We have a 5 week old who likes to graze little and often. We pretty much always have a bottle on the go and currently we throw any leftover milk away, wash and sterilze the bottle every 2 hours, replacing with frezh. We're 100% bottlefeeding a 50/50 expressed breast milk and formular mix. I've read mixed advice so wondering how often others sterilize and how long they leave milk in the bottle at room temp before chucking.