Calculating Radius of electric field

[Blackhawk4560]

Question: Is there a way to calculate the radius of an electric field? By that I mean, say we have a cathode on one side, how far away can the anode be? Is there a way to calculate this? I'd imagine it would need to be experimentally derived due to the number of variables (shape of object, vacuum state, voltage, amperage, etc) but physicists know a *bit* more about this than I do!

Thanks again for all the assistance, and I hope I put this in the correct topic-

 

[HomogenousCow]

The electric field in theory never vanishes no matter how far you are from it, so it depends on how weak it would have to be for you cut it off and just think of it as zero.