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.
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.