You should review your bids often, about once every two weeks.
For campaigns that use automatic targeting, review the performance of the automatic targeting defaults (i.e., close match, loose match, substitutes, and complements) in campaign manager and make adjustments to your bids to meet your campaign objectives.
For campaigns that use manual targeting, monitor for high-performing keywords, categories, or products and raise your bids for a chance to increase your impressions.
You can use the Targeting Report and sort by highest to lowest sales to determine which targeted keywords, categories, or products are meeting your business goals. For targets that generate conversions that meet your business goals, consider raising your bids. You can also identify keywords, categories, or products that are not meeting your conversion goals and consider lowering your bids for these targets.
We recommend that you limit your changes while testing strategies, so that you can attribute the difference in performance to a specific change.