In those old clunky freight elevators? I have no idea. But probably longer than you or Walt think.
Jarman and Norman heard a police radio give the location of the motorcade and knew the motorcade would probably arrive at the TSBD in about 5 minutes...
The motorcade was at the location given in the broadcast at 12:25..... And they departed immediately for the fifth floor....That would have had them passing through the first floor by the Domino room at about 12:26 / 12:27....