UA 811 (cargo door blowing off) has been retired too, although I believe the actual aircraft (N7413U) stayed in use for several more years.

Flight numbers often get "unretired" after many years have passed. United flights 266 and 553 crashed 20-30 years ago, for example, but both are active flight numbers today.

I could imagine the required time period to pass before a number is unretired would be "as long as people remember the numbers". In the cases of UA 93 and 175 this may be forever.

Often numbers immediately below or above the retired number will be canceled as well, because flight numbers are usually assigned in pairs. For example, United currently has no flight 585 (737 rudder failure near COS), and no 586 or 587 either. But UA 92 and 94 apparently still exist, as do 174 and 176 -- all are the reverse flights as the lost aircraft.