How long to hold key combinations when booting a Mac?

In general, you should hold the key(s) down until they take effect. For single-user and verbose modes, that means hold them until you see text start to appear. For the others, it's generally ok to let go once you see the Apple in the middle of the screen (the Apple is drawn by the booter, which loads after the firmware has detected startup options).

If you have FileVault 2 (full volume encryption) set up, things are a little more complicated, since some key combinations (mainly single-user, verbose, and safe boot) don't take effect until after you've authenticated to unlock the startup volume. It currently seems to do a good job of detecting the key combos before the authentication screen, then having them take effect after you "log in". But if that's not working for you, try holding them down as you press return on the FV2 authentication screen, and keep holding them until you see them take effect.

That's because it will differ slightly depending on what hardware and the startup behavior it's being preformed on. From Startup key combinations for Mac it states:

Some Mac features are available by holding down certain keys during startup.

Press and hold the keys immediately after you turn on your Mac and hear the startup sound. Keep holding until the described behavior occurs. These combinations work with Intel-based Mac computers.

So the timings will very a little bit although it should be relatively quickly to realize the target behavior it taking place.

As an example my 7 year old MacBook Pro does everything considerable slower then my 2 year old MacBook Pro so to give explicit timings for various startup behaviors will vary as much as there are different speeds of processors, etc. and is why Apple states it in the terms that do and not not in seconds, etc.

Make sure you hold down the special key(s) upon hitting the power button, and keep holding the special key(s) down until you hear the startup chime.