From Longman Dictionary of Contemporary Englishit’s (only/just) a matter/question of timeit’s (only/just) a matter/question of timeused to say that something will definitely happen at some time in the future, but you do not know when I’ll find the key eventually. It’s just a question of time. It’s only a matter of time before we catch the person who killed her. → time
Examples from the Corpus
it’s (only/just) a matter/question of time• But they believe it's only a matter of time before the disease crosses the county boundary.• 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. 