From observing external galaxies, you get, on average, one approximately every 30 years in a galaxy about the size of ours. You can't see them in ours because most of our view of our own galaxy is obscured by dust and gas. Estimates on the size of our galaxy are 100 - 400 billion stars. I used 250,000,000,000 as an average size for calculation purposes. The cluster has about 10,000,000 stars ie. the cluster is 25,000 times smaller than our galaxy, so supernovae should occur 1/25000 as often, so 30 years times 25,000 = 750,000 years on average.
Cool!
I was just curious. Makes good sense.