On the possibility of lead acid battery recoverability
Failure:

When Lead Acid battery cell voltages fall below nominal voltage , the chemistry of plate oxidation (sulfation) accelerates , which is an electrical insulator layer that results in internal resistance, Ri or ESR. Over months ,it may be recoverable if done without damage, 

This promotes cell imbalance which results in the weakest cell degrading faster and overcharge cell voltage while series of cells is in spec. Thus rapid outgassing  might occur in that weakest "over-voltage" cell during charging.

Prevention

Apply saturated charge to prevent sulfation; can remain on charge with correct float voltage. Pulse charging can also help with prevention during use and during gentle recovery.

If outgassing and sulfation was excessive, then recovery is not possible due to acid contamination.

Analogy

The analogy is smoking with lack of daily exercise, being lazy and vascular buildup of calcium carbonate (hard oxide). (hardening of arteries)  Even if you have no symptoms of pain. Fix it now, before it is too late. Don't ignore it for next 6 mos. Risks are variable, but results are inevitable.