When to divide perennials depends on the type of plant and how quickly it's growing. You don't have to wait until your perennial plants begin looking like doughnuts. In fact, it's better if you don't. Keep an eye out for clumps that have grown 2-3 times their size within 2-5 years. Any over grown clump or any clump that has simply exceeded the space allotted is a candidate for division.
Spring is usually the best time for division, since the plants are actively growing their leaves are not so developed that the root system can't take a little disturbance and still feed the top of the plant. However, just as different plants can go different lengths of time before being divided, some plants, like peonies, prefer to be divided in the early fall.
In the photo here, the daylily has increased in size 3 times and once it is fully grown this season, it will be growing into the neighboring daylilies.
