Can you tell me roughly how long it would take 2x16 floor joists to begin to rot in an unvented crawl space, 2" over a dirt pad. The joists are holding up a Florida room addition to my home. I live in northern Delaware.

There are too many variables to give you an easy answer. Amount of moisture present- proper amount of airflow-type of wood and its quality, etc.
The crawlspace should be vented. There should be a layer of polypropylene (min. of 3 mils) placed over the dirt (gravel to hold it in place). Keep all water away from the crawlspace. Do this and the joists should outlive you.