First, there are two different rewards.
1) Coins you got from every NH you give.
2) Supplies you get as a bonus if you return a neighbor's NH within a window.
There are two different effects.
1) There is a cooldown between being able to give consecutive NH to a given neighbor, of 23 hours. You can't help City A every hour, and City A can't help you every hour.
2) There is a window of time after a NH has been given where, if the NH is returned, the returner gets a bonus of Supplies in addition to Coins. This is usually also 23 hours. Expanding that window means you could get more Supplies, as returns that would have been too late originally would now qualify for the bonus Supplies.
For example, picture you doing a full map NH. The cooldown prevents you from doing it again for 23 hours. If a neighbor sees the notification that you NHed them, and they NH back, this means the 23 hour window would start immediately, but would only be exposed for minutes after the cooldown ends. Normally, you would only be able to get the bonus Supplies if you manage to return the NH within minutes of the cooldwon ending. By expanding the window, you could do full map NHs almost every 2 days and yet be able to get bonus Supplies from any neighbor who NHed you over those 2 days.