From wiki:

With its 0.22-second attack rate and the max channeling time, the Death Ward can attack up to a maximum 37 times. Can deal up to 318.18/477.27/636.36 (Talent 522.73/681.82/840.91) damage per second to a single hero.

I’m tired of getting people playing Witch Doctor who don’t understand that Maledict + Death Ward is not necessary to kill a support. Death Ward is likely to kill a full HP enemy support in 4-6s, making Maledict not only unnecessary but also increasing WD’s mana costs and making it easier to cancel than by using a simple Q->R combo. Or alternatively, if you are playing WD and have an ally core going 2v1 on a enemy support, it is likely that Q+E with the damage output of your ally is enough to kill a enemy support without needing to use Death Ward.

Learn to preserve mana and to not put skills on cooldown when they are not needed.