It depends on the model. On some of them, especially on those with OLED displays, it is possible only in the activity mode, in order to prevent burning in the display (besides draining the battery too). So if it is the case with your watch model, and you need to have the display constantly on for a longer period of time, enable the display option "always on" for Activities, and start any activity (for example Cardio).
And if you need the display visibible 24/7, then you better change the watch for a model with a passive MIP display. OLED displays are not suitable for such use.
