Run the forward process long enough and the result is indistinguishable from pure noise. The schedule is what decides how long "long enough" is: at every step it says how much of what is currently there survives.
Take a toy schedule in which every step keeps of the signal present at the start of that step, replacing the rest with fresh noise. At step the image is completely intact, so the fraction of the original still present is .
After how many steps does the fraction of the original image still present first drop below ?
Give a whole number of steps.