It is actually posted directly on the NLI website, which says letters of intent contain this provision:
"[h=1]Statute of Limitations[/h]
I am subject to the NLI penalty if I do not fulfill the agreement; however, if I do not attend an NLI member institution to fulfill the agreement or penalty and four years has elapsed since my signing date, the NLI is no longer binding. Therefore, this NLI is in full force and effect for a period of four years, commencing with the date I sign this NLI, if I do not attend an NLI member institution during the period of four years."
Link to NLI website, which is linked from NCAA.org
See also, athleticscholarship.net, which is a website designed to help athletes understand the recruiting rules and to protect themselves:
https://www.athleticscholarships.net/what-is-the-national-letter-of-intent.htm
"Provision 10 – Statute of Limitations
[FONT=Arial, Helvetica, sans-serif]This section says the entire contract is void after four years if you do nothing. Do not worry about the section; there are several appeals, releases or penalties that can be served in order to get passed the NLI before a four year period."[/FONT]
[FONT=Arial, Helvetica, sans-serif]I also found this on Al.com, but the article is from 2011 so the rules obviously could have changed (but based on my above links, I don't think they have changed):[/FONT]
https://www.al.com/sports/index.ssf/2011/11/college_athletes_rights_nation.html
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"The NLI is a one-year contract with a four-year statute of limitations, Peal said, but "the statute of limitations only applies if a prospective student-athlete does not fulfill the agreement. For example, the prospective student-athlete signs an NLI but does not enroll at the signing institution. The prospective student-athlete is subject to the NLI penalty for a four-year time period if he or she never fulfills the agreement. After four years has elapsed the NLI is null and void which means it is no longer applicable so the prospective student-athlete would not be subject to the NLI penalty."[/FONT]
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I also think that general contract law could apply. You cannot sign a lifetime deal that provides no valuable consideration in return. For such a deal to be enforceable, it would have to be clear on its face what you were signing....and the mere fact we are having this conversation proves the NLI does not obviously contain language that it is enforceable in perpetuity [/FONT]