Then said I, Lord, how long? And he answered, Until the cities be wasted without inhabitant, and the houses without man, and the land be utterly desolate, And the LORD have removed men far away, and there be a great forsaking in the midst of the land. But yet in it shall be a tenth, and it shall return, and shall be eaten: as a teil tree, and as an oak, whose substance is in them, when they cast their leaves: so the holy seed shall be the substance thereof.
The question “How long?” means how long would God’s judgments continue on His people. The answer was “Until the cities are laid waste and without inhabitant, the houses are without a man, and the land is utterly desolate”. God will spare a remnant, a tenth, but even this remnant will have to pass through deep tribulation. This remnant, “holy seed”, is like the living stump of a great tree that survives after the rest of the tree has been destroyed.