I have played SF a long time ago and I have always tended to use the "half-moon" type moves. I have always avoided the charge characters, well, because I can't seem to use them properly.

The issue I have is using the moves in quick succession. A good example would be Ryu vs Guile.

With Ryu, I can really spam hadoukens fast.
With Guile, I can do sonic booms at about half the speed of the hadoukens.

Is there a special technique to doing charge moves quickly? How long do you need to be holding the "charge" direction for?