There are many different types of air fresheners available. Depending on what sort of deodorising you need to perform, you can choose from one of many freshening techniques. Determining which is the longest-lasting will depend on what odours you are trying to eliminate and in what environment. Freshening the air inside a car, which has a small cabin, is easier to do long-term than deodorising a larger room that may be connected to a building's ventilation system.
Gel-Based Air Fresheners
Gel-based air fresheners contain the air freshening chemicals in a gelatinous material which, when exposed to the open air, will begin to lose its moisture as the water inside the gel evaporates. The fragrance gets released over time as the gel dries out. Compared to a candle-based air freshener, the gel-based air fresheners can last longer, because they do not burn away and can be covered up and resealed when deodorisation isn't needed.
Aerosol-based Air Fresheners
When applied directly to fabric, such as furniture upholstery or automotive upholstery, aerosol-based air fresheners can last for an extended time compared to hanging air fresheners. The reason for this is that the perfumes and fragrances inside the upholstery are neutralised directly. An air freshener that simply hangs in the air is only changing the odour of the ambient air around it. Aerosol-based air fresheners can eliminate odour molecules that sit deep inside upholstery.
Air ionisers are becoming an increasingly popular way to deodorise and clean air in rooms. These types of fresheners use ultrasonic sound waves to eliminate odour molecules in the air. They are typically integrated into an HVAC system or placed in rooms as individual units, which consume the ambient air around it, neutralise the odours, then redistribute it throughout the room. The downside to ionisers is that they must be running to neutralise the odours. If powered indefinitely, these air fresheners will last the longest, but they require a constant power source in order to be most efficient.
Oil-based odour eliminators are becoming popular because of their longevity and efficacy. Using highly concentrated scented oils, the diffuser-based fresheners expose the oils to the ambient air, which absorbs the fragrance of the oil. Typically, these scented oils are strong enough to overpower most nefarious odours in a room. To make the oil diffusion-based fresheners more effective, simply place more of them around a room or area.