This is all personal preference. I set my timeline back 90+ days and if there are 0 impressions on them then I remove them. It's not hurting the account, however, it's mentally taxing to go through them all or filter them out each time. If you don't have any impressions for 90+ days then why waste that mental space? BUT, some keywords are seasonal, so I do give them 90 days to prove themselves before killing them. If you ARE getting impressions and no clicks, that's a totally different story. but it sounds like you're clear on that already. Good luck!