What is a good estimation for the limit of what a person can know?

[Hacker Jack]

For example: A person who gains degrees in 5 different fields. Will this person be able to refer to information learned in all 5 fields and bring it up at will?

Going by my own understanding of my self it seems to fade away when something new is learned and focused on. But once you revise over it it can be learned quicker than when you were first introduced to the information (maybe like what muscle memory is for exercise). I'm not sure about whether gaps of more than 10 years away from the information will allow you to revise and learn it quickly though. I am not sure how the brain stores memory, maybe I should ask in biology, but brain studies in general still seem to be a young subject.

 

[Halc]

It depends on if what is learned is put into use. Apparently much of what you learn in 5th grade is pretty useless in everyday life, hence the program Are you smarter than a 5th grader? which pitts children who've just learned various facts against adults who've put decades between them and the learning of the same facts.

But if you use apply the knowledge of those 5 degrees every day, you'll probably retain it, and better than in the academic days. Even so, most field require constant update courses to keep abreast of the state of the art.

 

[Klystron]

Some knowledge persists even as memory and ability fades with time particularly when one practices the related skills. I find that reading, writing and, to a lesser degree, speaking language helps retain rules of grammar and syntax, definitions and spelling, and language communication skills.

Mathematic knowledge and skill retention requires even more practice, as may be expected regarding math as an international, constantly evolving language. Deep knowledge of a field such as electronics seems to occupy a permanent niche aided by extensive experience with electronic theory, internals and devices.

One multi-PhD told me, "I cannot say how many languages I understand. When I hear or read a language I know; I respond." Another friend with doctorates in aero, physics and information theory said much the same about aircraft. Seeing a familiar airplane, picture or scale model recalls reams of data such as flight characteristics and design.Personal health, age and cumulative injuries affect memory even among the most astute.