Nevit's Voice is item which gives you 10 recommendation points in addition to 10 recommendations that you can use on other players.
The waiting time for donation starts at the last time you donated.
In addition, this waiting time is shared by all characters on an account, meaning that if you donate with one of your characters, all of your characters will have to wait 20 hours before donating again.
However, once you've obtained the item, there is no waiting time between use and you may use as many of them as you wish at the same time.