I lost a nice Ghost All Mountain Range MTB to a similar hairline crack.
While nobody can predict when or if it will break completely, the logical argument is this: there was a reason why the hairline crack appeared in the first place. This reason must, with a chance of 99.999...% be something that is not an individual event, but the systemic and constant wear and tear (material fatigue) caused by frame geometry and the dynamic forces acting at that spot. This material is, after all, not glass, where a single pebble impact can easily create hairline splits.
Assuming that it is so leads to the conclusion that the split will indeed get larger over time - no matter how long it takes. As the split has already weakened the area, it can be further assumed that the rate of growth will be at least as fast as before, maybe faster.
Aside of the technical/objective aspects, there is the statistical thinking - no matter how small the chance that the spot fails during a ride in a catastrophic way: the possible damage to the rider is very high, possibly fatal. Think about going OTB at high speed or in dangerous traffic, or over the side of a steep mountain ledge. So the expectancy value of the damage (probability multiplied by damage) can possibly be very high indeed.
Your conclusion depends on your character and risk aversity. I personally went and bought a much nicer new bike - it was a welcome excuse for that. ;)