Most of the perennials flower for several weeks only, average from two to four weeks. But there are exceptional perennials that flower from 4-6 weeks up to months. The best native & nativar performers are Agastache, Coreopsis - tickseed (Coreospis verticillata, Coreopsis grandiflora, Coreopsis pubescens), Echinacea purpurea (coneflower) and it's hybrids, Gaillardia aristata and hybrids (grow blanket), Gaura lidheimerii and hybrids, Hibiscus - cultivars of herbaceous species, Phlox paniculata (tall phlox and hybrids), Rudbeckia (black-eyed susan).From grasses are excellent Sporobolus heterolepis (prairie drop seed) or Panicum virgatum and it' s cultivars (switch grass).
Non-native perennials - Allium x 'Millenium', Achillea millefolium and varieties, Calamintha nepeta ssp. nepeta, Hemerocallis (reblooming varieties), Geranium x 'Rozanne' and newer cultivars derived from 'Rozanne' Kniphophia, Lavandula ssp., Lecanthemum x superbum, Nepeta x faasenii and cultivars, Sedum telephium, Perovskia, some varieties of Veronica.
For semi-shade to shade there isn't such an abundant choice, but nativars of Dicentra formosa 'King of Hearts, Dicentra 'Luxuriant', Dicentra 'Fire Island', or some varieties of Heuchera (e.x. Heuchera 'Berry Timeless') will work perefect (and from non-native perennial - Astilbe ssp.).
Generally long flowering times are not the main advantage of perennials, so for the best results always choose and combine different structured perennials (different textures) and count flowers as extra bonus. Also add annuals, biennials, smaller flowering shrubs or evergreens and bulbs.
With some plants close by the entrance or a patio is worth some dead-heading or shearing after the main flowering period is nearly over. Dead-heading can prolong flowering for several more weeks.