When a battery is in an excessively discharged state, it does not readily accept a standard charge. The battery may appear to be accepting a charge, but charging is occurring only at the surface of the plates.
With an AGM (sealed lead-acid) battery, higher voltage is required to get the job done. Charge the battery using a charger that can consistently supply between 18 and 20 volts. Typically a charger like this is only found in the service department of a motorcycle shop. The service technician should test the battery prior to charging, and again after charging is complete. Charge time will vary based on how bad the battery is discharged. Final testing will tell you if the battery has been recovered, and to what extent.
With a conventional (flooded) lead-acid battery, slightly higher than normal charging amps are required to recover the battery from an excessively discharged state. This higher rate could be up to 10 amps, but no more. The battery may get warm during charging, but this is a good sign. It means the battery is charging. Make sure the battery's water level is up to the top line before charging, and monitor throughout the charging process to make sure no additional water is lost. If so, replace water as needed.
If the battery still does not hold a charge after going through these procedures, best bet is to purchase a new battery. In order to ensure that the new battery does not suffer the same fate, make sure battery is charged properly and frequently enough to keep it from draining to an excessively discharged state. Charging once a month is recommended in most cases. More frequent charging may be required on today's vehicles that tend to have a constant electrical drain. Typically, electrical drain can come from a radio, clock, alarm system, or other computer memory.