Hi expert NB's!

The enemy-debuff (Minor Maim - reducing targets damage by 15%) for 4 sec due to Shades' attacks from using NB's Dark Shades: since the shades' attacks last at least 15 secs, does Minor Maim lasts 15 + 4 = 19 secs?
Or does Minor Maim only apply during the first 4 secs of the shades' attacks?
Thanks for your help!