How far back in time can you challenge a will?

any ambulance chasing attorney will be happy to take your case.....for a fee. 

Win lose or draw, he still gets his fee. 

Most charge just for a 10 minute consultation, just to see if you even have a case. 

From what I know of wills,  there is a certain  time period  to challenge a will, and once that period passes, there is nothing to stop  the allotment of assets based on the last legal will.  50 years is surely well past any reasonable claim to challenge. 

Even if you came up with extraordinary damning new evidence.....the matter is probably legally closed,  with no way to overturn the decision. 

But  if you care to try.......be prepared to hear the same thing from a lawyer. 

The issue is not when it was written.  It is when it was admitted to probate.

Until your grandfather died, he had the legal right to change his will.  As such, no will is really official until after a person dies and his will is admitted to probate.  

When the will is presented, the probate court will review the will and decide if it is facially valid (i.e. is properly executed and witnessed).  At that point, it is tentatively admitted to probate and the time starts for filing a will contest.  While the exact time frame differs from state to state, a will contest must be filed before the time expires.  

You will need to consult with a probate lawyer in the state in which your grandfather lived to see if it is still possible to challenge the will.  From what you are saying, I am doubtful that you are still within the time frame, but, if your grandfather just died, you would be within the time.  However, there are some specific requirements for a will contest, and your question does not include sufficient details to determine if you have a valid basis for challenging the will.

Difficult - very difficult given the time that has elapsed.  And you will need more than suspicions about the grandsons actions.  Best to hire a lawyer but that will cost you money.