First of all, the question I would ask is what kind of wax are you using? The biggest misconception these days is that the term 'wax' is used synonymous with 'sealant'. Traditional waxes are made of natural Carnauba wax from Brazilian palm trees. These waxes can last from 2 weeks to 6 months on average depending on the brand. If you don't know the exact durability, you can ask the product manufacturer regarding their longevity. Sealants, made from synthetic polymers, last 1 month to a year depending on brand and quality. Coatings, made from ceramics, can last from a year to indefinite amount of time.
However, one of the biggest factor that plays into durability is how often do you drive it and under what conditions. If it's a daily driver plowing through the highway, your wax, sealant or coating will last for a shorter time span. Other conditions lowering the durability are weather and environmental contaminants. Parking under the sun may cause damage to the protection but nothing could be worse than parking under a tree and having a bird bomb eat away your protection along with the car's clear coat!
Another factor is maintenance. Your wash method can compromise your car's protection. There are horror stories of how several automatic and tunnel-of-doom car washes can not just eat away your protection but also cause deep scratches and swirls all over your car! Without a proper maintenance and wash method(eg. two bucket wash method), no amount of protection can save your car.
The question here would be to ask - how do I know when my car's protection is compromised? The simple(and complication-free) answer would be to see how water beads and sheets off your vehicle's surface. Car waxes, sealants and coatings usually have hydrophobic(water hating!) properties which get consumed as time passes. So, take a spray bottle with water and spray it on your car's hood. Then, observe.