Azure AD - How Long Must a User Hold a License Before I am Charged for That User?

Hi dlacombe 

When you buy out the licenses you are already paying for the service.

• Licenses added in the middle of your billing period appear on your next invoice. If you pay annually, you are invoiced within a month for these changes.
• On your next billing statement, the previous charge for the original number of licenses is deducted. We add a prorated charge for the time period with the original number of licenses and add a charge for the new license count. There's also a charge for the current license count for the remainder of your billing period.

It's worth to take a look to understand the lifecycle of the billing:
https://docs.microsoft.com/en-us/microsoft-365/commerce/licenses/buy-licenses?view=o365-worldwide#how-does-buying-or-removing-licenses-affect-my-billing-statements

Hope it helped.