[ppcsussp] Galactic black hole merger

How frequently do two galactic black holes merge somewhere in the Observable Universe? We can measure their gravity waves. Spherical distribution?

It would be interesting if the frequency were of a human timescale, and if we lived in a universe where gravity were relatively stronger and gravitational waves from distant black hole mergers caused macroscopic effects visible on earth. Once every few years, say, spacetime will unpredictably distort because of an event in a distant galaxy billions of light years away.