Answer: Unfortunately, I can't think of any perennial that will be able to bloom for such a long period. Most perennials bloom for just a week or two. Black eyed Susan or Rudbeckia fulgida is a long bloomer, as would be purple coneflower (Echinacea purpurea) and Nepeta "Walker's Low" (not a small plant despite the name) will rebloom repeatedly if cut back periodically.
For a good fall display, you might try the sedums. These go through a season-long growth and development that is quite attractive, beginning early in the spring as small sprouts and growing tall, then flower buds developing from green to pink or red and finally opening in late summer or early fall. The long lasting flowers can then be left to dry on the plant through winter as decorative seedheads and trimmed off the following spring.
All of these do well in full sun and a well drained soil and are relatively drought tolerant. (The sedum is the most tolerant of dry soil.) They all attract butterflies. They will do best if planted in soil that is deeply prepared with organic matter added to it, and kept mulched with several inches of organic mulch year round.
I hope this gives you some ideas.
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