It is possible to challenge the validity of a US patent by having a
third party make a request of the US Patent Office to reexamine an
issued patent in light of new information, prior art, etc.  I believe
that this is called a reexamination request by a third party.  There
are different rules for reexamination, but the rules I care about are
for pre-November 29, 1999 patents. My questions is, once such a
request is filed, how long does it typically take for the patent
office to review such a request?