Perennials come back year after year giving you many years of enjoyment and only need to be maintained.  There is no set time to divide them but there are guidelines you can use and the plant will give you signs that it is time to divide.  Flowers will be smaller and have fewer blooms.  The center of the plant will die out leaving growth around the edges.  Or they have simply out grown their area.
A good rule to follow is if it blooms in the spring divide in the fall, if it blooms in the fall divide in the spring.

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Plant

Allium  
Artemesia (Wormwood) 
Aster
Astilbe
Bearded Iris
Bee Balm
Bergenia
Blanket Flower
Bleeding Heart
Candytuff
Chrysanthemum
Coneflower (Echinacea)
Coral Bells
Coreopsis
Croccus
Daffodil
Daylily
Goldenrod
Grape Hyacinth
Hosta
Joe-pye Weed
Liatris
Lilies
Obedient Plant
Peony
Phlox
Rudbeckia
Salvia
Shasta Daisy
Spiderwort
Veronica
Yarrow (Achillea)

Time

1-2 years
2-3 years
2 years
3-4 years
2-3 years
3-5 years
3-4 years
2-3 years
3-5 years
2-3 years
2-3 years
2-3 years
4-5 years
3-4 years
when overcrowded
2-3 years
when overcrowded
3-4 years
when overcrowded
5-8 years
2-3 years
3-4 years
3-4 years
2-3 years
3-5 years
4-5 years
3-5 years
rarely / 5-6 years
every other year
2-3 years
2-4 years
3-5 years

Season

Fall
Early Spring
Spring
Spring
Fall
Spring
Spring
Spring
Early Spring or Fall
Early Spring
Spring
Early Spring
Spring
Spring or Fall
Fall
Fall
Fall
Early Spring
Fall
just after emergence
Spring
Early Spring
Fall
Spring
 Fall
Spring before growth begins
Spring
Early Spring
Early Spring
Spring
Spring or Early Fall
Spring