As we understand it, the extent is infinite. However, isolated charges have a hard time staying isolated. They tend to rearrange the charges around them, so tha

As we understand it, the extent is infinite. However, isolated charges have a hard time staying isolated. They tend to rearrange the charges around them, so that the net effect of their charge becomes effectively zero if you define a vicinity for other particles. I believe the term for this is "shielding."