Has it always taken three or more years to achieve black belt status?

No, not at all. As a matter of fact many of the pioneers of karate came back from overseas with little more than around one year of time in training to open dojo and teach karate to Americans. On average black belt has been awarded only after learning the actual movements with out bunkai or bunkai that was simple and incomplete, i.e. just enough to explain why we did kata and often not much more in the year of duty most served overseas.

It was assumed, rightly so in an Asian culture, that one would begin dedicated, diligent and comprehensive training and study over the coming years. In other instances the dojo and masters who actually had contracts with the military overseas felt it necessary to present black belts at the end of an tour or possible lose students. The military in the fifties vied for black belts and would be disappointed if they didn't achieve those levels before heading back to the states. After all, the majority would not return for additional duty overseas, i.e. only a hand full went back for successive tours or extended tours of duty.

In some rare cases for those pioneers they would actually receive a certificate for sixth dan or level of black belt with the understanding they would not assume that level until they had fifteen years or more of continuous diligent dedicated practice. Many donned the rank upon return to the states and opened dojo.

This may have been the precursor to the McDojo phenomena we experience in today's world of martial arts. Sometimes practitioners would attend just long enough to look good then branch off and open their own dojo and charge a premium fee. With appropriate marketing and con artist type promotions toward unsuspecting and inexperienced individuals it worked. McDojo's were born and have flourished since.

In defense it is turning around where more and more dedicated individuals are diving deep into their studies and bringing the true essence and intent of martial arts back to the forefront and through the internet more and more are no longer entering into practice without the fundamental knowledge and understanding necessary to make informed choices and decisions.

I personally applaud such folks I am aware of such as:

Michael Clarke of Shinseidokan dojo; Iain Abernathy Sensei; Marc MacYoung; Patrick Parker Sensei of Mokuren Dojo; Lawrence Kane and Kris Wilder Sensei of Martial Secrets blog; and many many more who have indirectly influenced my continuing studies in the art of karate-goshin-jutsu-do.