According to Lava Lite, LLC, you can leave lava lamps on for up to 10 hours straight. Other testers have said that you should really turn them off after eight hours. Give it a rest for several hours before turning it back on.
Warning Signs It Has Been on Too Long
If the lava starts to form small "bubbles" that float to the top of the lamp, then the lamp is too hot. This may happen even if the lamp has not been on for 10 hours. If the lava falls to the bottom in a large lump, then it is much too hot, and at this point may not be fixable.
Keep an eye on the lava lamp. Do not allow it to get too hot, as it can burn you and ruin the lava lamp. If you want continuous lava lamps running, purchase several lamps to run on a continuous cycle.