There is not an exact length of time assigned to leave the unit running, but much like other appliances in your home that generate heat, we recommend that it not be left unattended, especially if it shares a space with pets or small children. If you’re going to bed, it’s best to give the fireplace a rest too.
The best answer to this is going to be unique to your home. Most residences weren't built yesterday and may not be up to code. While the fireplace insert is perfect for a cozy evening at home, it should not be left running indefinitely or used as a primary heat source. If you plan to use your electric fireplace for more than a few hours at a time, consider having a dedicated circuit put in. Standard US receptacles can support 1800 watts and should not run at more than 80% capacity on continuous loads, or more than 1440 watts. At 1400 watts, the fireplace insert requires a large amount of energy; if other items are in use simultaneously AND for extended periods of time, be careful not to overload the circuit.