When a result is only a matter of time, then it is inevitable. The only variable is the amount of time that will pass before the event occurs.
For instance it is only a matter of time until a person dies. It will definately happen, but the temporal location is unsure.

This expression is overused. Often times events that are declared only a matter of time, are really only likely. However, things are so rarely sure, that if this phrase was strictly used, it never would be.

However, one could always say that anything that can happen will happen, so every situation is only a matter of time.