Google Ads is billed monthly, starting from the day that you purchase your campaign. Google Ads gets automatically renewed on the same day each month, unless you cancel.
If you decide to cancel, then the campaign will run until the end of the month that you paid for, and then stop. You can always reactivate the campaign again later, without having to go through the setup procedure again.
If you decide to downgrade to a less expensive package, then this will take effect from the next billing period. The current campaign will run until the end of the month that you have paid for.
If you upgrade to a bigger package, then your current campaign will be replaced with the new campaign, right away. The new campaign will run for a month, from the day that you upgraded. This means that the monthly renewal date will change to the day that you do the upgrade.