It's suggested to use a thermostat, but what wattage is your heat mat, I found that a small 5watt mat can be left on 24/7 just as long as you have some substrate like reptile carpet. Any higher wattage I think would get too hot if left on 24/7, if u have a temp gun you can check temps after leaving it on for a hour or so and see if the temps are suitable.
Leave it on 24/7 but definitely regulated by a thermostat or dimmer switch. Aside from the facts that most heat pads get way too hot when they are unregulated, heat pads have been known to malfunction at times and get hot enough to melt themselves.
An on-off thermostat is $30, and a dimmer switch is even cheaper at about $7. It's worth it for the safety of you (in case of fire hazard) and your gecko.