Melavah Malkah means escorting the queen. Therefore the sooner one eats Melavah Malkah the better. However, if one is not capable of eating right after Shabbos one may delay the meal until later, however not later than midnight (Shmiras Shabbos K'Hilchoso 63:5, Yechave Da'as 4:25). Kaf HaChaim 300:14 holds that it should L’Chatchila be eaten within four provisional hours after sunset. I have heard that there were some Chasidishe Rabbis that ate Melava Malkah Sunday morning however I do not know which ones.