Frequency of gravitational wave detection
Harry (2009) cited several different sources stated that, as far we know, the rate of detectable events will be


40 neutron star mergers per year
30 10 M$_\odot$ black hole mergers per year
10 neutron star/black hole mergers per year


This is within a radius of about 200 Mpc. This cannot, however, be used to extrapolate the total rate of such events, because of detection bias - the more massive the objects, the more easy they are to detect. The same thing happens with exoplanets, but for different reasons (e.g. planets that are more massive or closer to their stars are easier to detect via transit or by radial velocity methods).