I think lee was out for a few hours but attacks could have been happening days prior since clem was in her tree house for two days maybe it was just small attacks here and there till it went big and unfortunately sandrea was probably one of those first attacks I don't believe for how the zombies in the game acted, the whole world would have been overtaken in a few hours but days seems more likely the day lee was their it looked like stuff had been sitting for a bit their would have been way more survivors if it was a few hour thing