I've done the drive from Ely, NV to Fallon, NV on US 50: 257 miles without encountering a traffic light. I'm sure this can be topped.
Topped by another Nevada drive...
The farthest I have gone is 398 miles. Driving from Las Vegas, NV to Fernley, NV (en route to Reno).
The route was the following:
* US 95, beginning at SR 157 NW of Las Vegas (where it downgrades from freeway to divided highway) and traveling through Beatty, Tonopah, Hawthorne (using US 95 Truck/unsigned SR 362 to bypass the signal in town) and ending at Schurz.
* US 95 Alt, from Schurz west to Yerington and north through Silver Springs, ending at a signal on Main Street (US 50 Alt-US95 Alt at SR 427) in Fernley.
Normally, I make this drive via Fallon instead of Yerington--this way has some higher speed limits and is three miles shorter. Doing it this way is about 395 miles, with the first signal still occurring in Fernley.
That route is the following:
* US 95 as described above, continuing north from Schurz to just south of downtown Fallon.
* SR 117 to bypass downtown Fallon (this cutoff bypasses several signals)
* US 50 west out Fallon
* US 50 Alt to head into the Fernley roundabout
* US 95 Alt from the roundabout, which encounters a signal just before the I-80 interchange.
The divided highway portion of US 95 along either route has two interchanges in a 46 mile stretch, so there's not any "regular" exits. Even if you didn't count that portion, it's still about 350 miles on mostly two-lane highway with no signals (flashing beacons on stop signs not withstanding).