As taboo of a subject as it is in most modern societies there really is no "normal" frequency of it... I suspect it is the same with birds. I have often wondered about Auggie (my dusky conure,) as he will do it MANY times each day... and I do mean MANY. Auggie seems to be the Ron Jeremy of the bird world.
I don't know if Auggie's virility is "normal" per se, but he has been going strong for some time and he is otherwise quite happy and healthy. I, however, don't know anything about the normal reproductive ages and development of different species of parrots. Perhaps Tex will find something on the age issue.
Parrot masturbation (much like human) can be a sign of any one of several things: sexual maturity and raging hormones, happiness/"over"-excitedness and closeness to the object of affection, or boredom. The first is virtually inevitable and healthy. The second is fine as long as the "object of affection" is appropriate. Boredom however can be a concern in pet parrots and should be addressed if you think this is causing the behavior.