I'm new here, so first I want to say "hello everyone". Having made my introduction I guess I'll jump right in:
The total charge of the "DNA molecule" is not one set value. The charge would vary with the length of the molecule (i.e. the number of nucleotides). I would assume you are referring to double stranded DNA (you didn't specify otherwise), so just draw a base pair labeling all charges as appropriate (and of course negating those which would be used up in forming phosho-esters) and that total charge should be at least proportional to the number of base pairs plus -4. Of course this is also dependent on the pH of the solution that contains the DNA (If the pH is such that the hydoxyl groups on the phosphates are protonated, the charge would be less negative)...but at physiological pH, and ignoring a bunch of other variables (cations like Mg++, etc.), that could give you a good idea.
Anyone feel free to add/detract from the above!