QUESTION: 
How long are things tested before they are used on the Hubble for real? 

ANSWER from Rick Fredo on the 30th April 1996:
Naturally, for such an expensive and complex spacecraft, any new
equipment, computer software, operational procedures or configuration
changes are tested several times against ground simulators/mockups of the
HST. For example, when a new and improved version of the on-board
computer software has been generated, it is tested in the VEST (Vehicle
Electrical System Test) facility at Goddard Spaceflight Center. This
facility consists of engineering models and flight spare equipment that
are identical to actual HST equipment. Its like having another Hubble
Telescope on the ground, but without the mirror. The software is
rigorously tested to thoroughly exercise all aspects of the code before
it can be uplinked to HST.  Once uplinked and resident in the HST flight
computer, it then receives further testing before it can be used to
control and manage the HST on a regular basis.

We also do testing at other NASA sites to support HST operations. For
example, there is a Nickel-Hydrogen Battery test facility at Marshall
Spaceflight Center in Huntsville, Alabama that cycles
(charges/discharges) a set of batteries to emulate the operation of the
actual HST batteries.  These test batteries are composed of the same type
of battery cells as flown on the HST. We use this test facility to try
out new methods of operating the HST batteries before actually
implementing them with the actual HST batteries.  We can also learn about
long-term affects from accelerated testing on the ground and thus, are bet
ter able to predict and correct for failures in orbit.

The test and validation program for operating the HST is a massive
on-going effort. I have only given you 2 examples - there are many, many
more.