Set an amount to spend over the lifetime of an ad set with lifetime budgets. It is important to select accurate flight dates, as Facebook will pace delivery to evenly spend the full budget across the selected number of days. Any unspent budget from the previous day will be carried over and spread across the remaining days.
When creating new ad sets, set the lifetime budget and flight dates here:
You may also choose to edit existing ad sets. Switch between daily and lifetime budgets by clicking edit at the ad set level:
We recommend pacing to spend no less than $25 a day per ad set. This is because Facebook needs enough data (impressions & conversions) to determine the quality of the ad. Ads that Facebook has determined are good quality will continue to be delivered more, and vice versa for poor quality ads. Give your ads a chance to gain some momentum with a fair budget of at least $25 a day
Read more about Facebook's lifetime budgets here.