The length of time necessary depends on the state. For example, West Virginia is 10 years, Ohio is 21 years.
The theory behind the law is that if you don't care enough to actually know what property you own for a decade or two, then you don't own it any more. Possession is 9/10ths of the law. The law developed out of squatter's rights and homesteading - which you may have appreciated 150-200 years ago when some people owned 10,000 acre tracts of land.
http://www.state.wv.us/wvsca/jury/adverse.htm
One who seeks to assert title to a tract of land under the doctrine of adverse possession must prove by a preponderance of the evidence:
(1) That he has held the tract adversely or hostilely for a period of ten years;
(2) That the possession has been actual;
(3) That the possession has been open and notorious;
(4) That possession has been exclusive;
(5) That possession has been continuous;
(6) That possession has been under claim of title or color of title.
If I understand the situation, neither you nor your father knew the actual extent of your property for more than 20 years . . .?
You should NEVER buy property without a survey and at least a 40 year title search. Some people use 60 or 80 years just to be sure. If the person who is buying property has the title search and survey done, the attorney would have likely warned of any adverse possession problems with the property.
Of course, the government Courts established this doctrine, but they do not allow it to be applied to government property.