a matter of days, miles, pounds, etc.

a matter of (something)

1. What is important is (doing something). It's a matter of sticking with the program in order to succeed.
2. (Something) is the main or consequential issue. With your reckless spending behavior, it's only a matter of time before you go broke. I don't care about the money—it's a matter of principle.
See also: matter, of
Farlex Dictionary of Idioms.

a matter of ˈdays, ˈmiles, ˈpounds, etc.

a particular number of days, miles, etc., especially when this number is small: Don’t worry, it’ll only be a matter of hours before he gets back. It will only cost us a matter of a few pounds. Travelling by boat could take us a matter of weeks.
See also: matter, of
Farlex Partner Idioms Dictionary
See also:
  • a matter of (something)
  • be just a matter of time
  • be only a matter of time
  • it's only, just, etc. a matter/a question of time
  • stick to
  • stick to (something)
  • only a question of time
  • a matter of time
  • a question of time
  • be only a question of time