Totally agree with what RellikZephyr said.
If you got a really old car without synchros, GET ONE!
If you got a dogbox, SLAM IT and SLAM IT HARD while keeping the foot planted throught the firewall!
Basically, in regards to the shift itself, as fast as you can without breaking the box. On average between clutching, changing gear and engaging the clutch again will take bout .5-.7 of a second. A straight cut shift will take between .4 and .6 depending on how hard you do it and in drag racing, that's a fair bit of time after 3 changes.