After the new patch was released demons do not despawn when summoning another demon. So what I do when Im in a tight place is have a Voidwalker out already and summon another void walker right before you summon the next VW sacrifice your original and you'll be able to have 1 minutes worth of shield rather than just 30 seconds.
P.S The sacrifices do not stack so dont do them both at once & Healthstones help to when running out of a place you would rather not die in