A typical liquid based termite treatments are known to last anywhere between 2-7 years. That's a broad spectrum of range, but just like everything else in the world, the effective period of a termite treatment can vary. For example, if termiticide is applied by a untrained person with no experience, there's a great chance that termite control is based on luck. Also environmental conditions and pH content of the soil can affect the longevity of the termite treatments.
Popular termiticides such as Termidor will last up to 7 years with 100% effective impact. Other termiticides like Dominion brand will last 7 years as well. Some other products can last anywhere between 2 - 7 years. But if you have termite treatment done by a pest control company, they will usually tell you in advance what kind of termiticide they use. If they don't tell you, ask them.
So I think I can say safely a typical liquid based termite treatments can last 5-7 years with full effect.
How Long Does Termidor Last?
I mentioned briefly above that Termidor lasts about 5-7 years. But let me explain in more detail. On their website, they claim that the ongoing field testings done by the United States Department of Agriculture-Forest Service show that Termidor termite treatment (Termidor 80WG) lasted for more than 15 years.
But there are just too many variables to determine how long termite treatment lasts. For example, if your pest control contractor mixes termiticide or insecticide with wrong amount of water, the effectiveness of the treatment will diminish. Fortunately, most of the termite treatments used by the contractors come with detailed instruction on how much water to be mixed with.
Another fact that can shorten the duration of termite treatment is the experience of your pest control contractor. The most important thing when it comes to effectively getting rid of termites is to find right spots for applying termiticides. That doesn't come naturally. That knowledge is obtained only through rigorous training and experience from the field.
When you call a termite control company for a quote, some will tell you that termite control needs to be done every year, which is true. But that doesn't mean they have to apply termiticides every single year. Make sure to ask what they would do with monthly/annual contract. A reputable company will offer an monthly/annual service to come out to your house to check the affected area and to search for any new activities for prevention.
Lastly, when hiring a termite control contractor, make sure to ask what kind of termiticide they use and ask them how long the product can last. Then go online and check for the label of the said product to see how long it lasts.
Although I'm not a big fan of EPA, I must give them some credit on this topic. Pre-1996, termiticide companies had no obligation to reveal the years of proven effectiveness. On October 1, 1996, EPA released Pesticide Regulation (PR) Notice 96-7, which requires a mininum number of years of proven effectiveness and new labeling statements before it can be registered with EPA.