Well it only lasts like a fraction of a second. Like very small timeframe. 0.1 seconds or lower.
Ascendant simply extends the Debuff timeframe to 6 seconds. Even if it's Ascendant from hirling, the weaken applying Arcanist 20% ED per enemy is there but it lasts 6 seconds instead of a small fraction of a second.
Also Ascendant counts as an arcane DoT so the arcane attacks description applys meaning you can reduce enemies resists by 62.5% and enemies don't have any or barely any arcane resists.
Although having Ascendant on the rogue without hirling is totally an option.