As often as is required by the plot.
It will always be relative to the plot or the dragon pro/antagonist/npc. There are no hard rules that say a dragon must always be the alignment the Beastiary says it is.
depends on the setting really, In the pathfinder setting I haven't seen it but in Eberron a dragons color didn't dictate his alignment
I had a Silver "fall" and become a major villain in one of my games, as they are described as the paladins of dragons and are capable of falling.