Yes, SORACOM platform has a mechanism to disconnect your Air SIM session when the session is idle for about an hour.
Even though the exact duration and conditions are not disclosed, a given roaming partner will randomly disconnect your session if there is no communication to its cellular radio facility.
To Keep the Connection Alive
If it is necessary to keep your session for a long time, please make sure you keep the connection alive using technics like as sending keep-alive messages periodically from your device to your server.
Furthermore, a session may be disconnected due to cellular radio condition or other environmental reasons. So please consider implementing a mechanism to automatically reconnect when the session is disconnected on top of keep-alive messages.