Short answer: Hitler didn't think that.

Longer answer (based on Richard E. Evans, The Third Reich at War; German edition, pages 508-544 and Anthony Beevor, Stalingrad, pages 102-131):

During the summer of 1942 the German army aimed for the Caspian Sea, the Caucasus and the regions South of the Volga in general. One year after Operation Barbarossa had begun, Leningrad and Moscow were still not conquered, Moscow was even out of reach at this point. Hitler needed a prestigious victory and he thought he could get Stalingrad without to much hassle. Strategic and tactical bombing of the city started in August 1942 and the 6th Army of General Paulus started its assault on September 12th. On that date Paulus and Hitler met in Vinnytsia and talked strategy and time tables. The occupation of Stalingrad should have in given - in theory - Germany the full control of the Don-Volga region, which had the potential to speed up the march on the Caucasus Mountains significantly. General Paulus promised Hitler that the battle would be over and won within weeks (and definitely before Winter would come). On September 30th the 6th Army had taken control over roughly two thirds of the city, which led Hitler to state publicly that the battle would soon be over. There is no reason not to assume that Hitler was confident that the battle would be over before Winter fell.

Nobody within the German high command thought of the possibility that the battle for Stalingrad would last as long as winter, because they couldn't imagine that the Soviets had the necessary potential to mount a successful defence in the area and even more, sustain that defence successfully for half a year.