Your electric furnace has one filter on the return side where the duct connects to the furnace. Replace the filter monthly no matter what they look like. This will keep the duct/furnace and A/C evaporator coil clean. Opt for the cheaper version when choosing a furnace filter as frequent replacement is more important than the quality.
If you are diligent on your replacement there is no maintenance for the electric furnace.
Emerald Sheet Metal Ltd.HVAC Contractor