[idiom] a question(=matter) of time

Meaning :
You can say "It's only a question of time" before saying what you think will happen in the future.

For example :
It's only a question of time until we run out of oil, so we're going to need other forms of energy to power our cars.
If you keep on spending money like this, it'll just be a question of time before you have a serious debt problem.
It's only a question of time you will be a grandmother.
It's just a matter of time my English

Note :
This idiom is almost always used with the word "only" or with the word " just", as in "It's only a question of time before...." or "It's just a question of time."

Song :
Brook Benton "It's a matter of time."