The tool tip on my XP stones states they will expire when update 15 is launched in August 2012.
I had fully intended to use my stone for my third life TR but a combination of laziness and a strange new desire to inflict bodily harm on anyone working for my internet service provider mean I might not get the chance.
I won't let the stones simply expire however. One of my characters *will* get to use one, but I'd really like to know how long I have to finish my TR grind. Because logging to find update 15 has been announced and gone live, and deleted all of my stones before I had a chance to use it on a different one would be irritating.
tl;dr: When is U15 coming out?