A complete answer to this seemingly simple question is more complex than expected, as life span varies among species. Within a species, life span may also depend upon latitude, time of year, and even local weather conditions.
Adult Life Span
Marking studies suggest that winged adults of many (and perhaps most) species live only a week or two, and that the male tends to live a few days fewer than the female. However, the adult life span of some generations may be much longer. In some species, adults that emerge in late summer or early fall hibernate overwinter in a sheltered spot. Also fall-hatched monarch butterflies migrate south in fall and northward in spring. Adults of these species may survive for 8-9 months or longer.
Total Life Span
Total life span includes time spent in the larval and pupal stages, as well as the adult stage. Each species description in Butterflies and Moths of North America includes the number of annual "flights" for that species. A flight is a generation of adults. Thus, if a species has "two flights from May through September" it means that one generation will emerge from the pupal stage in spring and a second in summer. Actual months of emergence depend on latitude. Life spans of these two generations will be very different depending upon the species’ strategy for getting through the winter.
If the spring flight comes from eggs that were laid in fall by the previous year's summer flight, the total life span for the spring flight is 10-11 months. Eggs laid in May/June by those adults develop much more rapidly, due to higher temperatures, and adults emerge in about 2-3 months, resulting in a total life span of 3½-4 months for the summer flight, or less than half that of the spring flight. However, if the species is one in which adults of the summer flight overwinter, then the spring flight develops from eggs laid in spring, and in this case the summer flight is the longer-lived generation.
Not all species have two flights per year. Some species, particularly northern ones, have only a single flight annually, or a total life span of about a year. Some Arctic butterflies are believed to have a 2-year life cycle due to the extremely short growing season and the scarcity of high quality food for the larval stage. And some desert species, which normally have a life cycle of only one year, may hibernate as larvae or pupae for up to 7 years waiting for adequate rainfall to ensure growth of the host plant. On the other hand, southern species may have numerous fast-developing but short-lived generations each year. Finally, among the many species that are distributed over a wide latitudinal zone, it is not uncommon for northern populations to have one or two flights annually while more southerly populations have many flights annually. In some cases, the number of flights is considered taxonomically significant; for example, the Eastern and Canadian tiger swallowtails are now recognized as separate species, partially based on the fact that the Canadian species has only one flight per year vs 2-3 for the Eastern species.
Average Life Span
Often people want to know the "average" life span of a butterfly or some other species. This is a very different question than the one answered above, as it requires knowledge of age-specific death rates. These are not known for free ranging Lepidoptera (or indeed for most wild animals). About all that can be said is that only a minute fraction of larvae survive to adulthood, and the average butterfly life span or life expectancy is correspondingly much shorter than the figures given above would indicate.