Richmond and the surrounding area can go for 2 years without any snow. Most years, we only get about 4" per event. BUT, on occasion, we get from 14" to 24" in one storm. When that amount of snow starts sliding down the roof, it takes gutters with it, and people can get hurt.

 Some installers put a staggered row of guards just up the roof from the gutter. This will allow snow halfway up the roof to slide down, sometimes from near the ridge. This wad of snow and ice can tear snow guards from the roof along with the gutter.

 I much prefer 2 staggered courses on the roof, about 6' or so apart. I don't use any particular measurement, I just put them on according to how I think that particular roof needs them, and with regards to how it looks on that roof.

 These are some examples my work.