Why does my Fenix flashlight step-down in output from turbo?
The step-down feature depends on the environment you are using your Fenix flashlight in. If it is hot outside, the light has internal and external heat coming into play. When it is cold outside, that helps keep the light cool. The reason it drops down is for safety purposes. If you have it on Turbo and put it down (forgetting it is on), it could heat up the surface why it is lying. It just keeps you from inadvertently leaving it on a super high setting.
If you are using it and you want to keep it in Turbo, you just tap the mode back up to Turbo. When it steps down, it drops to High. It will repeat that pattern as you are using it on Turbo. There's really not a set time for cool down. The step down feature is more of a warning to you to ask you, "Do you want to keep it in Turbo?" The light drops down and then if you cycle back up to Turbo, that is telling the light you do want to operate it in Turbo. So it's always best to think of your environment and use the light at the output mode that fits the size you are illuminating.
It's also possible that the battery in your flashlight is low. When the battery loses energy, it may not be able to sustain the higher brightness levels. Try a different battery in the flashlight or try recharging the battery.