In one session, around 40 h. The worst case of not sleeping was during a military excersice however, designed to take us to our limits and beyond. 7 days of camping in the forest, attack and defending, digging potholes and having lessons about tellermines etc. We got a total of 10h of sleep during that week... By the end, to stop moving was to fall asleep while standing and eyes open. The level of coherent thought from us was 'just follow the backpack in front of me. Just follow the backpack and everything will be fine.' and we were honest to god starting to hallucinate and see dreams while awake.
After that particular excersice, back at the barracks, we all slept 12 hours, were awakened for food and then given 24h free. Most of us slep at least 18h of that...