when using mana burn or manabrake on another character, the MP loss does not visually register on the victim's character display. if the player no longer has enough mana to cast a spell, he or she simply does nothing even though it shows up as them having full MP.
furthermore i had talked to leonis during pegasus testing about the extremely long duration of soul wreck at max level (8 seconds). if i remember correctly he did mention that this skill duration should not scale with skill level. i also believe that the tooltip simply lists the duration as 3 seconds.
Thanks~