I personally had found that whole bit a bit of a stretch, it just so happens that it's the Enterprise of all ships being constructed in Riverside, Iowa. I had always assumed they were built in space, for a wide range of reasons, mostly from the onscreen evidence that ships were done as such, and the massive amounts of energy it must take to break the atmosphere from the ground. Most people I know seem ready to accept this, saying that the original plaque said San Francisco, therefor it could have been built on the ground in San Francisco. As for construction times, they never really said how long they take but I would imagine the larger ships take a few years, mostly from a common sense standpoint, and the evidence seen on screen (otherwise, would the Dominion War really have been that much of an issue?). As for Columbia NX-02, I think it took around two years or so, as it was pretty far into construction when Archer saw it in...March of 2153, but I could be wrong. Enterprise NX-01 took from 2149-April 2151 when it was launched and wasn't totally done yet, so that's about two to three years, depending (dialogue suggest that the ship had some trial runs prior to it's commissioning). Anyway, my whole thought was that it probably started earlier then 2255, or what we saw was just the space frame and the outer shell and the rest took until 2258 when an official comissioning was apparently pretty close (Pike's words about that being a reward for a safe return). On the other hand, considering the stuff Starfleet and the Federation can do technological wide I can see why some might think it's a streatch that, especially by the 24th century, would take a very long time. I think it does however, considering the heavy losses they take on from the Borg and the wars that followed, and the fact older ships were seen in service still. The Enterprise-D I believe was meant to have a long, drawn out design and construction history, but evidence seen onscreen in TNG suggest otherwise as final components were apparently designed and constructed a year prior to launch.
I've kind of gone on a rant here, oops...