According to Osho, his discourses last ninety minutes because audience alertness naturally drops: after 30 minutes a third sleep, after 60 minutes two-thirds, and by 90 minutes all—so he leaves. He adds, humorously, that without a wife to enforce the 'KISS—Keep It Short, Stupid' rule, a time limit protects against endless talking.
He stops at ninety minutes because people doze off by then, and without someone to make him keep it short, a fixed limit keeps things concise.