The threshold value goes from 1 (fully white) to 0 (fully black). If the value is higher than 1, it will be white. If you change the maximum time, you have to change these values too:
If you’re using a plane, you can do it in the level BP or in any custom BP you want. I’d create a separate blueprint for the plane if you need to use it more than once. This screenshot is in a widget, hence the dynamic material and stuff, but you can see the same setup in a level BP in my first answer.