The frequency of your charges depends on your billing threshold.
When you first start running ads on Quora, you will be charged when you hit your billing threshold, or on the first day of the following month. Whichever comes first.
The first threshold is $25 USD, and rises with each successful charge to $50, $250, $500 and $750*. The same value thresholds apply in all currencies (converted to the local currency).
If your threshold charge is successful, your threshold will increase and you will not be charged again until you meet your new threshold, or on the first day of the next month.
If you spend below your threshold by the end of the month, your unpaid balance will be charged the first of the next month.
*Thresholds are equivalent to USD $25, $50, $250, $500, and $750. Thresholds are subject to change based on currency exchange rates.
An account with a $25 threshold starts a campaign on January 1st.
If the campaign spends $25 on its first day, the account is automatically charged $25. This charge will appear as a "Threshold" payment on the Billing & Payments page. The account's billing threshold will also increase to $50.
If the campaign spent $20 between January 1st and January 31st, the account would receive no charges in January because $20 is below their billing threshold. To cover the unpaid balance from January, the account is charged $20 on February 1st. This will appear as a "Monthly" payment on their Billing History page.