I have never read of a CRSC application expiration date; I do not think there is one.
There is a six-year barring rule for retroactive payment however.
2017:
Here is how the Army explains the six year retro limitation:
"There is a Six Year Statute of Limitations: CRSC is subject to the 6-year statute of limitations, 31 U.S.C., Section 3702(b). In order to receive the full retroactive CRSC entitlement, you must file your CRSC claim within 6 years of the date of any VA rating decision that could potentially make you eligible for CRSC or the date you become entitled to retired pay, whichever is more recent. If you file your claim more than 6 years after initial eligibility, you will be restricted to 6 years of any retroactive entitlement."
Ron