Just as with any other type of footwear, the life of your hiking boot insoles depends on a number of factors, including how often you wear them, what kind of terrain you're exposing them to, and how well you take care of them.
With that said, most insoles can last anywhere from six months to a year before they need to be replaced - though, of course, this varies from person to person. If you only hike on occasion, or if your hikes are relatively short and not too strenuous, you can probably get away with replacing your insoles less often.
On the other hand, if you're an avid hiker who hits the trails often and for long periods of time, you'll probably need to replace your insoles more frequently.