It takes as long as it takes. Just remember that reheating hot food too many times is not good, try a plate warmer to keep food warm for longer. Try prompts, make sure the resident likes the food, try using fortified food to give the fuller feeling quicker, take resident into a quieter area as they may be distracted too easily, take all distractions off the table. Try and establish whether dining has become a habit ie the person is eating for the sake of eating (look for body language to signify enjoyment or suggest an activity right after lunch which may interest them and note the response). Time is something we all need more of, staff and residents, there is no easy answer.