To be kept on a charge continuously a "float" charge is needed. A float charger will deliver a lower voltage charge (2-v range) consistently and will can help keep a SLA battery in good working condition for long periods of time.
At a full 12-14v charge, the charger needs to be removed as soon as the battery is charged or the plates can become corroded and a battery can be ruined in days or weeks. Unregulated Transformer-Based Chargers, should be removed right away after a full charge is achieved.
For a float charge a more sophisticated charger is needed. If a unit is being unused for an extended period of time it should be stored in a cool place with a float charge on it.
The charge of the battery should be constantly monitored, making sure it is never fully discharged, and charged periodically with the charger being removed as soon as a full charge is reached.