As far as battery life is concerned, any lead-acid battery should never be allowed to sit in a discharged or partially discharged state for any length of time. If you have significantly used a battery for a session, you need to put it back on a charger immediately after use--even a single night in a significantly discharged state will cause irreparable harm to the battery.
As stated, all lead acid batteries (including SLA & AGM) will begin to sulfate as soon as the charging cycle is discontinued. The speed by which the lead plates are coated can vary, and the design of the battery will also determine how severely the plates are damaged. So called "smart-chargers" can prevent this process by applying a safe, low-current charge that is delivered at a voltage that is low enough to prevent cooking the battery. This is typically in the neighborhood of about 13.75 volts @ 500mA, but can vary. It is safe to keep any lead acid battery charging at this "float" level for extended periods of time, because the charge voltage remains below the gassing voltage that will depleat the electrolyte.
BTW - Heavily sulfated batteries are just door-stops, and cannot be reconditioned very effectively to be restored to deep-cycle usage. Therefore, it is a BAD idea to leave any lead acid battery idle for long periods of time.
