This light uses the NarsilM UI which you can find details about here
The default setting is called ramping mode where it starts off low but gradually brightens if you hold the button down. You can change it to a mode that has set levels if you prefer.
As for how much use you’ll get it will depend very much on how often you use turbo mode, but turbo is really only used to impress your friends. With careful use, making sure you use just as much output as you need, it should easily last several nights.