What is the longest the offering period can be?

The maximum ESPP "option" term (i.e., offering period) is 27 months for plans with a lookback feature but is five years for plans that use only the purchase date stock price (i.e., the up-to-15% discount is applied only to the end-of-period stock date price). Plans often provide for multiple purchase periods (e.g., six months) within a long offering period.