Because he was underestimating Finn AND basically playing with him.
Once he got serious about it, it DID last about a second.
From the Foster novelization:
Despite his fear, Finn raised the beam to defend himself. Ren lunged, struck—and Finn parried. Shards of light flew, illuminating the snow and the surrounding vegetation. Drawing back slightly, Ren considered his unexpectedly determined opponent, then resumed his assault with a vengeance.
Finn blocked him again and again, once letting the other man’s beam slide against his own and harmlessly off to one side. He counterattacked, to no avail. The longer the contest continued, the stronger Ren seemed to become. It was as if he was enjoying the challenge. Feeding upon it.
At least, it appeared so until Finn parried, swung, and unexpectedly stabbed, the tip of his lightsaber beam grazing Ren’s arm. That made it more than a challenge. Taking a step back, Ren reconsidered his opponent. When he closed the distance between them anew, it was with a purpose that had been previously lacking. Expecting an execution, he had found a contest. Now he had been touched. It was time for play to end.
Advancing relentlessly, he was driven by something that Finn could not even sense, far less counter. Still the ex-trooper fought back, until Ren landed a blow that cut across Finn’s chest and sent the lightsaber flying from his hand. It landed in the snow six meters distant.
It was over.
Leaving that aside, as you yourself noted,
Ren isn't exactly in top shape OR firing on all cylinders at the moment
Finn is a trained stormtrooper. A sanitation specialized one, granted, but still trained (and from the books, shown to be pretty good in his training marks).
Finn is a trained stormtrooper armed with a lightsaber. That helps, usually.
We don't know it yet, but there's an off chance that Finn ends up being Force Sensitive.
