mbf explains everything well. I will only add that I heard this in an online lecture (MIT graduate course on nuclear physics I believe) though the professor said he didn't actually do the calculation to determine how long that would actually take. But I would estimate it between 10000 to 100000 years factoring in all possible background radiation sources which corresponds to mbf's estimate. So maybe it's not as crazy as I was led to believe.

It's important to remember that cells are only really vulnerable to radiation damage when they are actively dividing because it's only in this stage that the DNA fragments are exposed and aren't attached to the 'scaffolding' used to securely store DNA in the nucleus. And when the DNA is stored it is readily repaired by the cell's repair mechanisms. (The DNA can still be repaired during cell division but the repair process is much easier to disrupt.) So you would have to concentrate alot of radiation energy directly into the stored 'DNA bundle', to cause enough damage to the DNA and the structure used to support it in order to overwhelm a cell's repair mechanisms. The only way to do this at this dose rate is to have the stored DNA hit by an alpha particle but the range of alpha particles from radioactive decay is so low that none would make it to your sample bacteria.

So if the bacteria are unable to move due to a lack of nutrients, and so not actively dividing, but are still at a temperature typical of their environment they would be incredibly resilient to radiation damage and you are unlikely to sterilize the sample.

Now if they were frozen it would be possible for the bundled DNA to accumulate enough damage over time to kill the bacteria once they were thawed but again not in any reasonable time frame with this does rate.

Fun fact: people who are cryogenically frozen will accumulate enough DNA damage from high energy cosmic radiation that they will effectively receive a lethal dose of radiation before being thawed if they are frozen long enough. Probably not the best financial investment over the long haul. #justsayin

edit: removed 'any reasonable time frame' from the second paragraph. / added 'at this dose rate' in the first paragraph.

It will not sterilize your sample since the dose rate is far too low. It will kill a few bacteria but most will survive and produces a new generation. It might lower the overall population a little for a short time but considering the adaptibility of bacteria to their environment it wouldn't take long for them to adapt.