When you sign up for a paid Keenai membership your account will automatically be renewed.
Auto renewals of your Keenai membership are on the anniversary date of when you subscribed. For example, if you sign up for a year's membership on 1/10/17 you will be billed again on 1/10/18.
If you happen to have extra time in your account when you pay for a membership (for example by adding an Eyefi Mobi card, or a camera credit), that time will not be used before your auto renewal kicks in.
In order to use the extra time, you will need to cancel the auto renewal feature of your account prior to your renewal date. Depending on where you paid, the process will be different.
Web app renewals:
Once your automatic renewal is cancelled, you will be notified prior to your expiration date to add your billing information again.