I Think the Longest I've Read Is 16 Days. The Only Way To Know For Sure Is To Track Your Temperature And See How Long it Is.
Here is from another site: "The process typically takes about two weeks, with 14 days considered normal. The follicular phase often varies in length from cycle to cycle; by contrast, the length of the luteal phase will be fairly consistent from cycle to cycle." A doctor who wrote one of the health pages for MedHelp here wrote that the luteal cycle is always 14 days; the quote above says "about" 14 days.