How long does it take to charge a car battery?

trying to charge a car battery from a Skoda Octavia. The battery wasn t fully discharged, and car was working okay except that the stop/start fuel saving function was not working. I suspected from what I have read was that it wasn t working because it is a taxi car, and the meter, roof sign light, taxi radio etc. is draining the battery and the alternator is not designed to cope with the extra load. This is now my second night charging it - the first night for about 8-9 hours, and now it s been charging for about another 4 hours, but the charger still says it s charging and the fully charged light has still not come on.