Repairing the monument ended up taking an entire day. Lex had to make preparations first, and when he was ready to work on it, he just entered everyones thoughts and gave them the idea that they should come back tomorrow, finally emptying the place.
No one realized that their collective idea to leave was artificial, or at least if they did they didn't let it on. That gave Lex plenty of time to fix certain things, such as the loophole that allowed people to make progress by constantly falling forward.
But after that, unfortunately, the fun time was over. It was time to get back to work, no matter how much he wanted to continue his shenanigans.
