I have let a battery go flat several times in a largely unused car. Several long runs failed to revive it.
I have had it on charge for a couple of days and the leds were all lit initially on my charger, eventually they started to fall but more recently have all lit back up again.
Battery age is unknown, is it goosed, I suppose the acid test would be to put it back in the car but I can't be arsed ending up scrounging a jump start on Sainsbury's car park.