A budget bid strategy needs at least 3 weeks of performance data for each campaign in the group to optimize plan performance. If you start a new campaign for which there is no historical performance data, manage the campaign manually until you have at least 3 weeks of data. Bid strategies typically take 2-3 conversion cycles for auction-time bidding or 2 weeks for intraday to move towards your performance targets. Learn more about allowing a bid or budget strategy time to learn.
Cold start behavior in biddable items
When you apply target CPA, target ROAS, target Impression share, or budget bid strategies (maximize clicks, maximize conversions, maximize conversion value) to brand new biddable items without performance history, the bid strategy applies what is referred to as a "cold start" behavior for the biddable items. The cold start behavior stops, once the biddable item has received enough clicks, or the item is older than 2 weeks.