I have been able to find a lot of discussion about the mechanics of the Storm Sorcerer's "Storm Guide" ability (such as whether or not it works against snow or if it works against magically-created wind/rain), but nothing about how long stopping the rain lasts or if it requires concentration (or to be awake, such as when the party is taking a long rest).

Storm Guide is a feature of the Storm Sorcerer from XGtE (p. 52; emphasis added):

At 6th level, you gain the ability to subtly control the weather around you.

If it is raining, you can use an action to cause the rain to stop falling in a 20-foot-radius sphere centered on you. You can end this effect as a bonus action.

If it is windy, you can use a bonus action each round to choose the direction that the wind blows in a 100-foot-radius sphere centered on you. The wind blows in that direction until the end of your next turn. This feature doesn't alter the speed of the wind.

Has anyone encountered this before? I imagine it would be of great use for a party while traveling/camping, but it would be really helpful to know how long it lasts after using an action to start it.

The Storm Sorcerer gains the Storm Guide feature at 6th level (XGtE, p. 52; emphasis added):

At 6th level, you gain the ability to subtly control the weather around you.

If it is raining, you can use an action to cause the rain to stop falling in a 20-foot-radius sphere centered on you. You can end this effect as a bonus action.

If it is windy, you can use a bonus action each round to choose the direction that the wind blows in a 100-foot-radius sphere centered on you. The wind blows in that direction until the end of your next turn. This feature doesn't alter the speed of the wind.

The ability to change the direction of wind near you has a specified duration, but for the ability to stop rain falling near you, the feature only says you can end the effect as a bonus action; it doesn't say how long that benefit lasts if you don't choose to end it.

How long does Storm Guide stop rain from falling near you? 

I imagine it would be of great use for a party while traveling/camping, but it would be really helpful to know how long it lasts after using an action to start it.

I have been able to find a lot of discussion about the mechanics of the Storm Guide feature (such as whether or not it works against snow or if it works against magically-created wind/rain), but nothing about how long stopping the rain lasts or if it requires concentration (or being awake, such as when the party is taking a long rest).