This sort of has 2 questions... Under the description I see no duration anywhere in either the standard players handbook for Channel Divinity and also in the Volume of Forgotten Lore book for this ability as well.
Starting at 2nd level, by presenting your holy symbol and spending a use of Channel Divinity, you can flood a 30-foot radius centered on you with magical shadows. All sources of light within the radius, magical or otherwise, are extinguished, and light coming from sources beyond that radius is dimmed: bright light becomes dim, and dim light becomes darkness. You and any creatures of your choice within the area, however, can see in this area as if the lighting had not changed. You can concentrate on this effect as though it were a spell. Each round you concentrate, you can also spend a bonus action to expand the area of reduced light by an additional 15 feet. Once you cease concentrating, the area of mystical shadow lingers until the end of your next turn.
Can I theoretically channel this forever? If so, can I continue to channel it until it covers basically a whole town given enough time? So doing so at night would basically wrap the entire town in pure darkness.