Question: 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.

Answer: 
### Your critical hits are not affected at all!

If you roll a 20 on an attack roll, meaning the die itself shows a 20, it is always a critical hit, as explained on PHB p. 194:

> 
> If the d20 roll for an attack is a 20, the attack hits **regardless of any modifiers** or the target's AC. This is called a **critical hit**, [...]
> 
> 
> 

**The GWM damage bonus is also not affected**, because only the damage dice are doubled. You can still add your brutal critical bonus dice, but the +10 on damage does not change, because it is not a die.
See page 194 of the PHB:

> 
> If the d20 roll for an attack is 20, the attack hits regardless of any modifiers or the target's AC. In addition, the attack is a critical hit...
> 
> 
> 

Great Weapon Master has no impact on this. You could have a -30 on the attack roll and it wouldn't matter. You still hit and you still crit.

The only time this wouldn't be the case would be if you rolled with Disadvantage and thus would need to take the lower of the 2 rolls. To crit with Disadvantage, both dice rolls would need to be a 20.