Once you have paid and logged in, you should stay logged in for the entire time that your account is active, unless you manually log out.

There is however a bug we're working on  -- if you don't have enough free space on your device, your device will automatically delete your login credentials and log you out.

This happens a lot with iOS, and also sometimes with Android.

We're working on a permanent fix for this, but in the meantime please free up at least 5-10 GB on your device and you should stay logged in for longer period of time.