The debuffs lasts 30 (or was it 35) seconds, refreshes/stacks roughly every 10 seconds outside the circle and will drop if you spend those 35 seconds inside the circle. Also, it doesn't refresh during Protect/Get Away/Hide, so if you're slow at flushing her out of stealth, stacks might drop anyway.
It also makes it annoying to have no precise timer for her abilities, as you can't really tell if you're supposed to refresh your debuff timer and she Hides two seconds later. Though maybe I'm wrong about that one.
Also, it's not 'melee range', it's 25-30yd from the boss? There's a lot of room inside the circle, which is exactly why Warlock gateways are so nice to quickly reach her during Get Away.