I have had mine on for 3 1/2 hours and the bottle is warm and the base is only hand hot the wax has not moved much yet just around the bottom of the bottle.
The wax on our smart astros is also very grainy, I'm hoping that it gets less grainy after they have run some more. I'm sure the instructions tell you to run them for no more than 6 hours. I've not left ours on any longer than that.
i have to say the colours it faze's thru are very good, i have mine on with lounge light on and its still pretty light, not as bright as a normal lava lamp but you can see it very well