If I have a bunch of keywords that aren't being shown, does this affect your quality score? When maintaining an Adwords account, should you "trim" off those keywords that haven't receive any impressions. What would be the minimum amount of days you would keep a zero impression keyword in your adgroup?
In my experience zero impressions has nothing to do with quality score. Normally I'll go from popular keywords to long tail, it allows to check 0 impressions words at the end: day 1: building 2 ad groups with 10-15 popular keywords. If OK, CTR, quality, let's go: day 2-3: another ad groups with less popular keywords, again CTR and general quality issue and so on...
Zero impressions are fine. It's when you have low CTR that your QS will be affected. So if you have impressions but zero CTR, that's trouble.
it will not effect your quality score but if they remain in your account long enough without no searches google will just make them inactive. If you are bidding on "rgaiofdsafdanvlarlgnalsdfg" as a keyword... google will just turn it off since its a waste of space...