If you sponsor a creator, you will be billed immediately upon subscribing to a tier - and subsequently billed according to the billing period of the tier. Most tiers are monthly, so you will be billed monthly for the tier amount.
If you are upgrading tiers, you will only be immediately billed the difference in the tier price. For example, if you upgrade from a $15 tier to a $20 tier, you will immediately be billed $5 for the upgrade. Your new billing date for the new tier will be the day of the upgrade.