Once your child shows both an interest in and an understanding of the concept of money - the fact that it can be exchanged for goods - they are ready to start learning the basics of money management. For many children, this may be as young as three or four. Their first allowance should be given at a minimum of once a week.
To decide how much, put several factors together: your children's age, how much their friends are getting, where you live--children in the rural areas of the country enjoy a lower cost of living than kids in an urban area like Miami--and how much of the total amount that you spend on your children you want them to start taking responsibility for. Talk with your friends who have children the same age, put the factors together, and experiment. You can always make adjustments later. You don't want your kids to be frustrated by too little or overwhelmed by too much.