In TNG "The Enemy", it took a modern Warbird five hours to cross the RNZ at supposed maximum warp...
Since the Romulan Neutral Zone exists, and a NZ has been mentioned in the Klingon context as well, we are well excused for thinking that the Federation always wants a Neutral Zone on all of its borders. That is, no clear-cut line where one nacelle can be on the Fed size and the other on the enemy size, but always a nice and thick buffer region that gives plenty of early warning about (uncloaked) invasions and spy activities and smuggling attempts.
Put another way, we have never really heard of a Federation border that would not feature a Neutral Zone. Except, apparently, the border against Cardassia, where there is no neutrality, just two sides brushing against each other but banned from holding armaments.
On the other hand, our TOS heroes typically deal with "the Neutral Zone". How can they tell which one, if more than one exist? Sometimes it's clear from the context, or perhaps from preceding but offscreen dialogue, but for example in ST5:TFF, it most definitely is not...
Timo Saloniemi