If you generate a graph with bacterial concentration (cfu/ml) vs
turbidity units, could you use this same graph in future experiments?
And when would you need to generate a new curve? What's the difference
between the graphs: turbidity (OD660) vs time and cfu/ml vs time?

There is no difference, the two graphs should be perfectly
proportional because the higher optical density is DUE TO refraction
of light by the bacteria, so there will be a direct correlation.  You
theoretically could use this curve again, however people generally
dont because slight factors (temperature, small errors in measuring,
etc) can affect it.  Usually, one creates a standard curve with the
mixture that will be measured, at the same time both the standards and
the experimental mixtures, and a new curve is made for each
experiment.

If you would like me to elaborate on this and get some sources to
better explain the procedure, I will gladly do so and post it as an
answer.  Please let me know if this would be useful.  Thanks!

Andrewxmp

There are acouple of factors that could lead to errors. The turbidity
is dependent on cell size, as well as numbers and the cfu value
assumes no clumping of cells and 100% viability. Since all three may
vary from culture to culture you cannot assume the relationship will
always be the same. However: for the same strain, grown under the same
conditions and for the same length of time, the results from one test
will be a pretty close match to the next.