If you want, you could leave your computer on 24/7 with possibly no problems. The first thing that would get damaged would be the hard drive, since it has to spin for such a long period of time. A dimming screen would occcur over a long period of time, 1.5 hours shouldn't do anything bad.
I leave my mac on all day, and put it on sleep when I go to bed. I rarely shut it off or restart.