From Longman Dictionary of Contemporary Englishit’s only/just a matter of timeit’s only/just a matter of timeCERTAINLY/DEFINITELYused to say that something will definitely happen in the future It can only be a matter of time before someone is seriously injured. → matter
Examples from the Corpus
it’s only/just a matter of time• It was only a matter of time before Lynn found out Phil's secret.• Your father is dying and there's nothing we can do. I'm afraid it's just a matter of time.• But they believe it's only a matter of time before the disease crosses the county boundary.• You'll learn how to do it eventually -- it's only a matter of time.• If he hasn't already killed somebody, then it's only a matter of time.• They think it's only a matter of time before he breaks. 