To find out when your upgrade plan expires, click on the "Manage Your Account" link on the Inbox.com Homepage.
At the top of your profile you can see what plan is active and when it expires.
When your Inbox.com Upgrade Plan’s expiration date is reached, your annual subscription is automatically extended and the fee is charged to the Paypal account you used when you subscribed, unless you have canceled the plan from within your Paypal account.