How long did it take for rattlesnakes to adapt without a rattle?

They haven't yet, that is a myth. Rattlesnakes still have a rattle and probably always will.

A rattlesnake's most distinguishing feature is its rattles, but baby rattlers don't have rattles until they shed their skin for the first time ... When an adult rattlesnake feels threatened, it coils, rattles and hisses all at the same time. Young snakes may coil and hiss, but you won't hear a rattling sound.