Negative keywords are an important part of effective campaign management. There are a few factors to consider when deciding whether or not you want to add in a particular search term as a negative keyword:
- Impressions: You want to make sure a particular term has had sufficient time to run and generate impressions. While 2-3 weeks should usually present you with enough data, that time period may not be sufficient if your ad has been ineligible or your product has been out of stock
- Clicks: Using your organic conversion as a benchmark, you can determine whether or not a particular search term has an excessive number of clicks. If it normally takes 25 page views for you to generate one sale, then you might consider negating a term that has generated 35 with no associated sales.
- Spend: Come up with your own "spend with no sales" limits based on your product category. If you are in a saturated market with a large number of sellers and search traffic, it is not going to take you long to rack up spend due to a higher cost-per-click.
- Relevancy: If a search term seems relevant to your product and is still not converting into sales, then you need to evaluate your product listing page. If your product listing page is optimized, you can also take a look at your competitors' offers, ratings and reviews.