Storing Exposed Slide Film-Undeveloped

Here's my question.
What is the longest that you have stored undeveloped film (Velvia and Provia-refrigerated) before processing it and what results have you gotten?
I know sooner is better, but I am curious as to what actual results people have gotten for long term storage.
Thanks in advance for any input.

Yes, sooner is better but I've never really tested it beyond a few weeks. I wouldn't worry about it too much though, National Geographic photographers work in pretty tough (hot, humid) conditions in a lot of places that don't have FedEx around the corner to take their film back to the lab. I don't know how long they carry their film but it's probably longer than that (much longer!) - and the quality of their photographs is still excellent.