Changes will reflect immediately when you upgrade your current plan.
But for downgrades, the time will depend on when you downgrade and the end of your current subscription plan.
For example: If your monthly/ yearly plan ends on 20th August and you request a downgrade on 11th August, then it will take 9 days to downgrade (which is the end of the current subscription).
You will still have access to your premium features until the end of your current billing cycle.