Used to express that something will happen in the near future, often with a sense of inevitability or warning. Can also mean 'even now' in certain contexts, but the primary modern usage is future-oriented.
It will start raining before long.
今に見ていろ、いつか成功してみせる。
Just you wait; I'll succeed someday.
If you keep living like that, you'll ruin your health before long.
Just wait, I'll show them before long.