Each time you authorize a new device, AeroFS generates a secure private certificate. The certificate is used by your AeroFS software to create secure channels for file sync and share between your devices. It is also used by the AeroFS identity service to ensure that only you can access your files.
Certificates are issued with an explicit end date, which is one year from the date of issue. Normally, AeroFS will automatically refresh your device before the current certificate expires. In some very unusual cases, your device may be unable to recertify automatically before the existing certificate expires. In these cases, the device will ask you to explicitly sign in.
If this does happen, you should not be asked to reauthorize that device for approximately one year.