In Inception, why didn't Cobb believe that limbo was reality?

Mal only confused limbo for reality because she had been there for fifty years. Just like the guys in Yusuf's basement who were spending 40 hours a day in dreams until the dreams became their new reality, she had spent more time in limbo than in real life an lost her hold on what had been reality. Cobb was only in their for a half-hour or so, and didn't have time to forget it was a dream.