Here's a helpful EPA chart that converts radioactive isotope activity to rem from drinking water

I have no idea about the EPA's methodology when converting activity to absorbed dose, but let's run with this info to see how many picocuries/becquerel we can consume before we reach the general population's 100 millirem dose limit per year. 

Source:  EPA

The chart above states values that yield 4 millirem per year.  To figure out our dose limit per isotope, all we have to do is multiply each isotope of interest's pCi value by 25, as shown below:

75 picocuries, or 2.775 becquerel of I-131 = 100 mrem/yr
2000 picocuries, or 74 becquerel of Cs-134 = 100 mrem/yr
5000 picocuries, or 185 becquerel of Cs-137 = 100 mrem/yr.

Each of these amounts will max out your allotted extra exposure from natural sources per year.  The total ballpark figure to not exceed is 500 millirem/year, but keep in mind - there are no safe exposures to radiation.  Every photon entering your body is a dice throw, capable of causing cellular damage that can lead to cancer.  You don't want to be paranoid of course, but you want to keep cumulative exposures as low as reasonably achievable.