It's widely reported that there were hundreds of kilometers of Viet Cong tunnels. However that may be hard to prove exactly, in retrospect, how long they were. OTOH a more recent source claims that there are 120 km of preserved tunnels.
More than 120 kilometers out of the 250 kilometers of the tunnels’ length has been preserved for tourism purposes.
A similar claim:
The Cu Chi Tunnels, stretching all the way to the Cambodian frontier, once boasted an impressive length of nearly 250 kilometers. Out of this, 120 kilometers have been meticulously preserved and are now open to the public.
Can this claim be verified with contemporary/peace surveys?
FWTW, Wikipedia cites a source for the 120 km figure... which doesn't actually give it. But it does give some area figures for the museum(s)/reservations. Which (after converting from hectares) seem to be only about 1-2 square km, in total. It's possible the tunnels have a high density, but this is inconclusive.