Fermi problem." You should be able to estimate an answer based on what you know and reasonable assumptions.
Michael Lynch has a crude estimate in his book The Origins of Genome Architecture. Without reading the book, can you come up with an estimate of your own? Is it larger than the circumference of the Earth? Larger than the distance to Pluto? Longer than the distance to the nearest star (other than the sun) or the the center of the galaxy? Would the string of DNA molecules stretch to the nearest large galaxy (Andromeda)? Or, would it be even longer than that?
In case you've forgotten everything you once knew about the structure of DNA, here's a brief refresher: The Three-Dimensional Structure of DNA.
You may assume that all of the DNA molecules are in the standard B-form with the dimensions shown in the figure.
I will not accept any answers in archaic measurement units like leagues, miles, yojana, or cubits.