According to the Sims Wikia page on Moodlets, the "Having a Blast" Moodlet has a maximum duration time of 8 hours. However, Sims with the personality trait "No Sense of Humor" will be able to have that particular moodlet last up to 15 hours. Also, toddlers, children and Sims with the "Childish" personality trait can only have that moodlet for a maximum of 5 hours. That time will be doubled for toddlers and children with the "No Sense of Humor" trait, i.e. up to 10 hours.
This kind of makes sense if you think that Sims without any sense of humor will be less likely to have fun, so it lasts longer when they do. On the other hand, childish and younger Sims have fun more easily, but get bored much more quickly.