Did you provide him a written commitment with an expiration date? If so rely on the expiration date. If you sent a commitment letter with no expiration date (not a good thing to do) then you have an open commitment.
If only a verbal commitment was given - then I would send a letter advising the applicant that he has X number of days to accept the commitment otherwise the commitment will be voided.
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The opinions expressed are mine and they are not to be taken as legal advice.