It probably makes sense not to leave them on for longer than 6 hours. Unless you have one with a dimmer that is. Helps keep the bottle in good nic too I guess.
Thanks I nornally only run my mathmos lamps for about 6 hours or less. I did leave one on one night when i was tired and feel asleep. Thanks bohdan for advise about the metallics i run mine now off a 30w bulb.
I know of one that was ran for at least 3 months continuously without problems. It ran somewhat hot (lava at the top) but never clouded up. That is certainly NOT recommended!!
I wouldn't run my lamps for more than 10 hours although I have done before (falling asleep) but to no ill-effect, as long as you don't do this too regularly. I read here once that an ex-member ran a Mathmos Telstar for 3 years continuously! All the lava turned white apparently.