My experience has been 24 to 48 months when printed on high performance cast vinyl depending on colors and sun exposure. Printing on intermediate, calendared film, I have seen as little as two months. The resins evidently get along a lot better with some films than with others.
We use the Duracoat UV/Abrasion Guard on all our outdoor prints and have not seen a print fade out since we started using it except for two different jobs both ruined by a detailer with an electric buffer. The damage wasn't immediately apparent but a loss of about 75% occurred withing two weeks. Duracoat estimates that using their UV/AG will add 12 to 18 months to the life of a print in outdoor use.