Although the components of an early stock compact are a bit marginal, running 24/7 is not out of the question. A key is keeping the mac cool. Don't obstruct the vents, and make sure that the ambient temperature doesn't get too high.
I've never seen a Mac truly catch fire, but I have seen them smoke a good bit. The four-pin yoke connector on the analog board can get pretty hot when it develops flaky joints. The cardboard cover on the foil side of the board makes intimate contact with the hot joints and can burn. I've been told -- but have not confirmed -- that the cardboard is impregnated with a flame retardant. In any case, I second Charlieman's recommendation for a checkup. I would freshen those solder joints (as well as those of the flyback) as a matter of course. Once that's done, I'd have few qualms about running a Plus all day (again, assuming that it gets adequate cool air). FYI, I now happen to have two on my office desk running continuously. The building is air-conditioned, so that helps a great deal.
I've never seen a Mac truly catch fire, but I have seen them smoke a good bit. The four-pin yoke connector on the analog board can get pretty hot when it develops flaky joints. The cardboard cover on the foil side of the board makes intimate contact with the hot joints and can burn. I've been told -- but have not confirmed -- that the cardboard is impregnated with a flame retardant. In any case, I second Charlieman's recommendation for a checkup. I would freshen those solder joints (as well as those of the flyback) as a matter of course. Once that's done, I'd have few qualms about running a Plus all day (again, assuming that it gets adequate cool air). FYI, I now happen to have two on my office desk running continuously. The building is air-conditioned, so that helps a great deal.