*Hubble needs 3 gyros to be able to be controlled in the three axes. IIRC it has 6 (for redundancy). So I expect that when it gets down to 2 working gyros, they can't control it enough anymore (this is what I mean by the 'last gyro' failure, not getting down to zero working gyros)
I wonder if they will use Hubble right up until it becomes uncontrollable. NASA decided to deorbit the CGRO while it was still controllable. There don't seem to be any concrete plans for sending a robot to Hubble any time soon, either.