When I was doing quite a few gardening lectures, I was often asked why perennials don't bloom as long as annuals. I had to explain that my understanding is that they bloom for a shorter time span because they need to conserve energy to return again the next year.
Most likely there is a much more scientific answer for this question but in my own garden I have had some perennials that bloom for a very long time and it seems like many of them don't reliably return. Hellebores though are quite hardy here.
This photo was taken last year on June 9th. You can see that the Hellebore was in full bloom. I couldn't find a photo of when the Hellebore began blooming but I'd guess by late March if not early April it was blooming. Since it certainly didn't stop blooming a day or two after June 9th, I can pretty much bet that it bloomed for over 3 months. That's an incredibly long bloom season for a perennial!
There are other perennials that bloom for a long time here on Long Island, I'll have to look through my photos and see what pops up. Right now though, I'm looking forward to seeing some Hellebores, Crocus and Galianthus (snow drops) appear any day now. Last year I shot a photo of a crocus bloom on February 20th.
What perennials do you find have a long bloom season?