Slower than the Mengelberg Andy Anderson linked to, faster than Bernstein’s dirge.
Mahler may have played it that fast, but geez, what does the author know about his own text?
As Curtis Lindsay said, it is meant to be a song, not the slow-mo ringing of the spheres. Embarrassingly in fact Mahler did make lyrics for it, as a love song to Yoko Ono… er, Alma Mahler.
In general though, and with the exception of this instance, Mahler’s own advice holds when it comes to conducting. When you sense the audience is getting restless—go slower. There’s a lot going on in Mahler: taking the time to let it play out is normally good advice.
It’s just that in the case of the Adagietto, there’s somewhat less going on. It doesn’t need that much time to play out.