it means: if we dont do something now, then who will and how long will it be before someone does something. It states that you should do something now, rather than letting someone else do something about it cause you don't know when that will be, cause when someone finally does do something it could be to late. For e.g. ebola virus & war
Don't answer this, but I'm doing what you do. I'm calming myself down Paul... I don't wanna shout at you, that's why I'm not answering your calls. Because I'm annoyed, so I'm calming down before speaking to you.