How long is the free trial and when am I charged?

Trial length depends on the plan you pick and is shown on the upgrade screen before you confirm, because Apple and Google set it per product and per region. Whether a trial is offered at all also depends on your store account, since each account can use a given trial once. Billing starts only after the trial has fully ended, and the store sends a reminder beforehand. Cancelling during the trial keeps your access until the trial period finishes and no charge is taken. See also: Is it possible to get charged before a free trial ends?