I agree with Nigel that 30 mins max. is about right. We often run the PJ early evening for the kids while they are waiting for their tea. When tea is ready the kids stop watching for usually 20 minutes or so and then resume watching. I never turn the PJ off during this period. I stop the player (ie. blue screen, not pause) and the player picks up from where it left off on pressing play.
I always put the dust cover on after use, as soon as the PJ has cooled off a bit. Mine isn't permanently mounted so when the PJ is completely cool, I actually pack it away after use, it's completely dust free then. OTT maybe but it only takes 3 or 4 minutes to set up when I want to use it and packing away every time serves another valuable function, it makes the wife happier, relatively speaking
Cheers
Croc