Around 300 mph. As someone pointed out, slightly faster than the high tech "blaster bolts" in Star Wars, but no where near bullet fast. Still, at that speed a spell effect travels the length of a football field in two seconds, so it's plenty fast. I would expect that it probably varies a bit between spells with longer ranged spells typically faster than shorter ranged ones, but the actual specifics have never interested me.
I should note again that I play a homebrewed version of 3e, and one of my many changes is that I've reduced the maximum range of a great many spell effects. Many spells have reduced range and medium and long range are much shorter than in 3e RAW. The purpose of this was to increase parity between spellcaster 'artillery' and martial missile weapon users. You may want to vary your answer (and I might want to vary my own answer) if your spell ranges can be quite long.