Your credit card will be billed once every month, on or around the 1st of the month.

When you convert to a paid account, your very first charge will be for a pro-rated amount.

Example:

Let us say you converted to a paid account on April 10th and selected 5 user licenses, for a total price of $399/month.

Your credit card will be first charged on April 10th. This will be for the pro-rated amount of (20 days / 30 days)*$399 = $266.

After the initial charge, you will be charged $399 on the 1st of every month, starting with May 1st.