This excerpt taken from the AAPL DEF 14A filed Apr 16, 2007.
Participation in an Offering. Shares are currently offered under the Purchase Plan through consecutive offering periods of approximately six months that generally begin with the first business day of the second and fourth fiscal quarter of each year. To participate in the Purchase Plan, eligible employees must authorize payroll deductions in whole percentages up to ten percent (10%) of eligible earnings, including, base salary, overtime pay, and commissions. Once an employee becomes a participant in the Purchase Plan, the employee will automatically participate in each successive offering period until such time as the employee withdraws from or is no longer eligible to participate in the Purchase Plan.