Even though California isn't thought to have the heat to produce high quality grapefruit, I've always wanted to try a Duncan and let it hang on the tree for as long as possible. I have heard of some people who have let Pummelos hang on the tree for over 2 years. Who knows what could happen letting a grapefruit hang for 2+ years on a mature tree. In general, I think California's cool nights may enable fruits to hang longer than other environments. Axel, over at the cloud forest site, lets his Cara Cara navels hang for two years.
-Brett