All partial billing periods will be prorated and paid usage won't begin until your trial is over. If you sign up for a premium product partway through the month, your first invoice will be prorated.
For example, if you sign up on May 25th, your trial will start on May 25th and will run for 14 days, until June 7th. Your charges and usage will accrue from June 8th to June 30th, and your first invoice will be issued during the first week of July.
Updated 9 months ago