Your daily budget is the maximum amount you're willing to spend each day to promote your Page. For example, if you choose a $10 daily budget and run your ad for 7 days, you'll be charged up to $70. Each day begins at midnight, so if you create your Page ad at 8pm, your first day will only last 4 hours before your daily budget resets for the next full 24-hour day.
You can choose to run your ad continuously or choose an end date. If you run your ad continuously, be sure to stop your ad when you're happy with the size of your Page's audience or you've reached the total amount you want to spend.