[nonbel]

A lot of this information seems to be available, for example figure 4 here[1]. According to that, the horizon distance for "a binary black hole system with the same observed spin and mass parameters as GW150914 for optimal sky location and source orientation and detected with an SNR of 8" was 1.5-2 Gpc. Now we only need an estimate of the rate at which events with these properties occur, not using the data from GW150914. I presume this is somewhat (a few orders of magnitude) less than the rate at which the mergers occur in general.

[1] http://arxiv.org/abs/1602.03844