if the subscription is being paid leave them alone.

if there's no sub i would be okay with
Under Lv 30 - 6 months
Above Lv 30 - Be within one expansion of the current one. So if a Lv 50 character quit before HW and hasn't paid their sub since, their name should go back into the pool when 4.1 is released. if they paid anytime during HW even if they didn't buy HW, they should be fine until 5.1.