You do not say what kind of butterflies you are seeing nor where you live, but monarch butterflies, (characterized by their brilliant orange and black colors) are known to migrate to the same place, at the same time, every year. They follow a rather rigid schedule, flying, lighting, feeding and resting at the same times daily. They can live for 6-8 months.
Tracking projects have been going on since the 1950s. Researchers have attached little sticky spots to the butterflies, enabling them to see how far the butterfly has migrated as well as how long they stay.
Other indigenous types, ones who do not migrate, have life spans which range from about 4 days (spring azure) to 10 to 11 months (mourning cloak). Females generally live longer than males.
There is a terrific special from PBS called, "Alien Empire." A link is below. Hope this helps!