It's extremely hard to put a figure on it.
The Anaconda has the longest jump range (somewhere around 41/42Ly in a bare-bones build?)
An Engineer can boost the Optimised Mass by 50%
Secondary effects can boost this stat even further (not sure by how much - probably at least 5%)
Another stat "Max Fuel per jump" can also further boost range.
Reducing weight will boost it further. In a bare-bones build you'll have stripped lots out, but some Core components (like the distributor) can't be removed, can be very heavy, yet can be made lighter through secondary effects on other mods. Because they're so heavy, a few % reduction of their mass via a Secondary effect can make a big difference.
I wouldn't be surprised if an Anaconda could do a 70Ly jump, 140Ly with FSD injection. Possibly more again in 2.2 with the ability to supercharge your FSD with stellar ejecta.