The current state of the art in remote measurement (which is in its infancy; the first papers to apply basic signal processing on measurement over IP networks are just coming out) suggest that the timeable thresholds are in the tens of microseconds over a WAN.

But that doesn't matter, because over switched "local" networks (read: from one end of a data center to another), the thresholds are in nanoseconds; everything is timeable. Attackers will spend the $30 to get a machine in the same hosting center as you as soon as the other attacks get less trivial.