Does one have to introduce a "speed of spell" to explain the spell blocking? IMHO one has to build up protection before the impact - so how can a wizard react to a spell that is in progress?
The "Protego" spell (the Shield Charm) is given as the foremost personal defense spell; it shields the caster, rebounding minor jinxes back to their caster. It's apparently fast-casting and is the bread and butter counter to a hex, jinx or curse, much like Stupefy (the Stunning Spell) is the bread-and-butter offensive spell (it's unknown which would win; I don't think anyone in the books ever did anything but duck or dodge Stunners). It isn't perfect, though; it must be cast well or a spell can break through it, and the big no-nos, the Unforgivable Curses, are unaffected by a Shield Charm.
There are magical artifacts that have been imbued with shielding spells; the Weasley twins offer several items of clothing in their shop that would shield the wearer like a Shield Charm would, without having to cast it. There are also certain racially-based protections; giants, trolls and dragons are unaffected even by Stunners unless they're strongly cast and well-aimed.
The problem is (as far as we know) a spell has no mass, so it's speed would not depend on the physical strenght of the caster, and we cannot calculate its speed by measuring how long it takes to cross a given distance, for all we know spell volcity can vary according to spell category and indivduals. Speed could even vary according to the distance of the target. If it is pure energy, we could suppose it travels at the speed of light but that is not realistic, wizards would literally need lightning fast reflexes. Plus, the flash of light we see (thought some spell don't produce light, transfiguration and charms especially) may only be a side effect of the spell, or a "tail" only in this case you would see the flash after hearing the thunder.
This is purely speculative but given human reflexes I'd say around 250 kmh as fast as the fastest tennis players can send a ball (world record 263kmh).
It's mostly a matter of anticipating the spell about to be cast at you... and having a good footwork!
Protego can be used as the shield charm in all situations except strong magic or the unforgivable curses (avade kedavra, crucio and imperio.) Also, some people say that you turn your wand in a horizontal position when you have to block. It might be true though. I do not get why people in harry potter just do the LCSAB technique to kill people. In other words, levicorpus which dangles the victim in the air, the cast the spell to your victim crucio which they would feel great pain, then use stupefy which will stun them and knock the over. Now you could use the killing curse and bombarda maxima and boom your victim is dead. Sucks how they did not think of that