How far the gravitational waves can travel? Does it depend on the mass which is converted to energy in a black hole/neutron star merger? If there is a finite distance, were we lucky to be in the range to detect these in 2015 for the first time?
In principle there is not limit to the distance that gravitational waves can travel. However, as the waves travel further and spread out over a larger area, the waves become weaker. The strain that the waves cause in a detector scales with the inverse of the distance that the waves traveled.*
Consequently, there is a limit to how far away we can detect a source of gravitational waves. (See also this question on the Astronomy Stack Exchange.) Now, were we lucky to observe the first event in 2015? The easiest way of answering this is by looking at gravitational wave observations since then (See Wikipedia's List of gravitational wave observations).
There has been a steady stream of observed events, and as the detectors have improved in sensitivity the rate of observations has gone up. From the event rate of later observation runs, we can work out how many events the detector would have been expected to see at the sensitivity it was running initially. This works out to seeing 1 event about every 2 months, which is consistent with the 2.5 events observed in the first 4 month run.
So, no seeing an event during the first run was (in retrospect) not particularly lucky. However, seeing the first the event almost immediately after the detector turn on, was incredibly "lucky". Moreover, in the catalog of all observed events, that first event still stands out is being one of the "loudest".
*Furthermore as the waves travel over cosmological distances they get stretched out by the expansion of the universe, they get even weaker over time.