The only thing I can find is this, and it's somewhat unclear.
Rested XP is based off a percentage of a level. If you log in and have a full bar of rested xp with your level 25 character, then swap over to a level 1 to craft for awhile, the rested XP will expire when you hit level 2! You MUST remember this in order to keep from wasting your rested xp.
I guess there are two questions we need answered. 1, what is the hard cap on rested exp? 100% of a level? 200% of a level? 250? 2, is rested exp based on total experience, or remaining experience?
If it's based on total experience, then it should be deducting from the total percentage over time (0 > 100 could take all of it, but 900 -> 1000 could take 10% of it).
But if it's based on remaining experience, then it is actually just addressing how much the time to next level you get buffed. Which would suck, and I hope this isn't the case.