Since children, and preschool programs, for that matter, are all unique, the answer will depend on your child's temperament and the type of preschool.  One size does not fit all and your child should start preschool when he or she is ready, attend for as many hours as suits your family needs, and enroll in a program that fits his or her temperament.
 
Many day care centers offer "preschool" programs for two year old children that only differs slightly from their programs for those two and under.
 
At age two, many children will be ready to follow simple instructions and start learning skills like taking turns or playing simple games.
 
Teaching more "academic" skills, such as a more formal introduction to reading, writing, and mathematics, usually waits until at least three years' old.  And many children are not ready for formal education until four or five years' old.
 
Preschool may be as little a few hours a week to a full-day program, five days a week.
 
Also, preschool education is not necessarily a requirement. Recent studies suggest that preschool does not have the benefits previously thought.  Students who do not attend preschool seem to do as well and perhaps even better than those who do.  Quality programs may, however, have certain lasting benefits.
 
If you are concerned about socialization, you may find that "Mommy and Me" or library programs fulfill that need.
 
Every family and child has different needs, so it is really up to the parents to decide at what age their child should attend preschool, if at all.
 
And with so many styles of programs available, from more traditional academic classes that focus on kindergarten preparedness, to music-focused classes, to programs that emphasize play, to "separation" programs (where the parent can leave when the child seems ready), to Montessori, to Waldorf, and everything in between, you should be able to find one that suits your child's temperament.
 
The important thing is that any program you select for your child should be a fit for your child and your family.