I sterilise all my bottles before i use them. I sterilise them and then put them together and take them out of the steriliser and store them in the fridge. I thought u had to sterilise everything before u used it?! Thats what im doing anyway.
With my first son we sanitized every time they got washed but that is near impossible unless you stay home. We ended up not using it but every couple weeks. We haven't used our sanitizer yet with our two week old but he has only had a few bottles when I've pumped to go out. It is not needed, but good to use sometimes, especially if a bottle got lost and sat for a long time with old milk or something.
I do it once every other day... I also try to wash the bottle as soon as shes done with hot water...