When can a virus modify DNA in every cell of a living organism?
The answer depends on two factors:


The type of vector (virus) you are using
How well your vector can be integrated. 


The bottom-line is that there is no guarantee the vector will work even if you start from the embyro. 

As far as I know, the problem of using vectors in general is that the vector you are using might not be integrated into the genome of the host. (Not necessarily get into the DNA, but just keep replicating during each S phase.) Therefore doing it earlier gives you a better chance to actually get them in the organism.

For example, there is a linear vector used in C. elegans that was injected into the oocyte even before fertilization. The cell later treats the vectors as broken DNAs and linages them so they could be pull to one side of the cell during mitosis. However, the huge chunk of vector DNA is mosaic. (i.e. only some of the cells in that worm have the vectors.) Other vectors like engineered HIV have better chance to integrate themselves.