Bids may be adjusted multiple times a day but will be adjusted at least once a day. We adjust bids as our models improve and change, which happens every time more performance data rolls in. We have various sources of performance data, but the most important are the performance reports we get back from the Amazon Advertising API, particularly at the campaign, keyword and search term level. We do not make time-of-day-based bid adjustments by default, but we have some dayparting strategies available in beta. Ask your account strategist if they might be right for your account.
Our default strategy is to maximize attributed sales while not exceeding the daily budget or target ACoS set in the app. Bids are set across all targets to achieve this goal based on a model of expected spend and sales of the overall set of campaigns. This means that some individual keywords or campaigns may have an ACoS higher than the target, but overall the campaign set will converge to a value at or below the target. We have maximum bid safeguards in place that take into account the marketplace and the price of the product.