There is no maximum length, but there is a minimum length of 800 words. If you have written 800 words and you have only reached up to 12:00, you can end your piece there, providing you feel your piece accurately copies Hersey and gives a good view of what your 5 people were doing in the morning, that there are switches back and forth between them. You do not have to use all the information or go up until bedtime, but you should begin and end for everyone at around the same time.