I sterilize after every use. It's totally overkill and I'm aware but when Dee was in the NICU it was common practice and I just got used to doing it and now I can't stop! I have the Avent bottle sterilizer so it doesn't involve boiling water and it's actually quite easy. I also have the Medela steam bags for her pacis.
Re: sterilizing bottles
I sterilized for almost 2 months but now just wash by hand with hot water and soap or put in the dishwasher.
I had a microwave steam sterilizer. It was the best.