Damage done to her by the champ who is cloned reduces the duration of the clone.
I've seen many people that don't know about this, and simply try to get out of the range of the clone asap, when they don't know that fighting her during this time is actually one of the best times to fight her.
Granted, this is easier said than done, especially when she has her ult. If the Illaoi is actually competent, she'll use the clone as bait for you, then just ult and kill you.
However, from my experience in low elo, most Illaois tunnel on the clone, allowing you to get free damage in.
Hope you learned something today!