There is no direct equation because beta CPM (counts per minute) is an electron count and (micro and milli) sieverts is an account of bodily damage.
Still, in the post below I site an EPA guideline dictating an intervention level at 300 CPM or 10 microsieverts an hour
So, let us for a moment presume that the two are roughly equivalent in a hypothetical scenario that allows us to speculate on the health implications of our current levels of exposure.
If that were the case, exposure to 700 CPM would be roughly 23 microsieverts an hour. That would mean that we in Phoenix yesterday would have hypothetically received 552 microsieverts in a 24 hour period.
In 18 days at 700 CPM we would be at approximately 10 millisieverts of exposure (there are 1000 microsieverts in 1 millisievert) just from exposure to air alone.
We also are exposed to radiation from the machines in our environment, radon gas, food, etc.
These numbers are not looking good.
The EPA recommends that our exposure to background radiation should not exceed 100 millirem a year, which is 1 millisievert, although we ordinarily are exposed to about 300 millirem in the US from background (3 millisieverts). Considerable background radiation derives from nuclear power plants, past nuclear weapons testing and radon gas.
There is no "immediate" health risk from 18 days of exposure of 700 CPM in the sense that people not will develop radiation sickness at this level. However, cancer rates and other diseases have been proven to increase at every 3 millisievert increment over 10 millisieverts. See my previous post.