When it comes to how long a DeWalt 20v battery lasts, there is no one definitive answer. Depending on the model of battery, usage conditions, and other factors, a DeWalt 20v battery can last anywhere from several months to several years.
In general, however, most DeWalt 20v batteries will last for around 500 full charges before needing to be replaced. This means that if you are using your battery on a daily basis and charging it fully each night, it should last you for about 1.5 years before needing to be replaced.
Of course, the actual lifespan of your DeWalt 20v battery will also depend on how well you take care of it. For example, storing your battery in a cool and dry place will help prolong its life, as will avoiding leaving it completely discharged for extended periods of time.
Overall, the best way to ensure that your DeWalt 20v battery lasts as long as possible is to simply take good care of it and follow the manufacturer's recommendations for storage and charging. With proper care, your battery should give you many years of trouble-free use.