The expected main sources of gravity waves are things like merging binary "star" systems where the stars are actually black
holes or neutron stars, and supernova explosions. However, these are relatively rare events.
So, for a gravity wave detector to see something, such an event must take place within the volume of space
where the detector has the sensitivity to detect something. That means for the original LIGO to detect something
we would have had to have been very lucky to have seen something.
With the upgraded version, the volume of space where LIGO will be sensitive is greatly expanded.
We have educated guesses for e.g. the occurrence rate of merging black holes. That can be used
to estimate how likely it is that a gravity wave detection would be made within a certain period of time
The current estimates give advanced LIGO a good chance of detecting something. (I'm too lazy
to check the actual numbers!)
So, if nothing is seen within a few years at the final sensitivity limit then people will have to reexamine
their estimates of event rates and/or general relativity.