That is like asking...
"How long will it take for two trains, one starting in New York, and the other in Los Angeles, to meet if the first is travelling at 60 MPH?"
How fast is the second train going, are they on the same track, or will they need to switch tracks, etc etc.....
We need more information to help you out, although Nitrates that low are no big deal.
Most people don't panic until they are well into the 20+ range.
What type of filtration, tank, inhabitants, etc are in each tank. Where are the tanks located in the house, near windows, in a basement, on the roof, or in the driveway?
You get the point. Anything can be effecting Nirates, but .3 is darn good.