While reading the Tràkata article on Wookieepedia, it said that that method of combat involves switching a lightsaber on and off to surprise an opponent. Yet it seems it takes a few seconds for a lightsaber to fully retract and then return. Is it an instantaneous opening or what?
I did a quick frame-by-frame and in ANH, when Luke first gazes on his new lightsaber (Anakin Skywalker's old weapon) in Ben's hut, the lightsaber pops up instantaneously. In one frame it's off, in the next it's fully extended. See the screenshots:
If you look closely you will notice that the characters jump slightly, so either some frames have been dropped or two different takes were joined in post-production. Whatever the reason for this, the end result is that the blade pops up instantaneously the very first time we are shown how it looks like and how it extends. At the very least it is faster than 33 milliseconds (the video I took the shots off had 30 fps).
In the film "The Phantom Menace", lightsabers take two frames to open;
In "Attack of the Clones", some sabers take as many as three frames to open fully;
In "Revenge of the Sith", the blades are back to fully opening in two frames.
Factoring in Bitmask's answer about ANH (and the relative frame-rate that the films were shot in), we can assume that lightsabers can take anywhere between .05 to 0.1 seconds to open fully.
It's worth noting that despite the fact that Luke's lightsaber appears instantly, when Obi Wan fights Darth, his saber takes considerably longer (almost a second) to unfold fully:
From what I know about how Lightsabers are made:
They are mechanical handles with a switch, inside there is a Power Crystal (Like a battery, it is what decides the colour)
So it would atleast take the time for a person to press the switch, then the speed would depend on how well it is made and how much 'power' is left in the power crystal. Imagine the Crystal to be like the lightbulb, and the handle to be the rest of a flashlight.
When it is a lightsaber Staff, it has two buttons, which need to be individually pressed to extend the light part, which would make it take longer to activate them both (Purely due to having to flick two switches). They still work of only one crystal.
This would indicate that crappy lightsabers are slower then good lightsabers. And as Jedi are meant to make their own lightsabers traditionally, it is an indication of their skill with precise crafting.
Of course in the movies it differs from each scene to match the best speed for dramatic effect.