As others have posted, you have until midnight the following day to cancel the reservation for a full refund.

NW actually has a better system than both CO & DL. CO & DL are 24 hours from the time you purchased the ticket. NW, on the other hand, gives you up to 47hrs58mins. You can purchase a ticket at 12:01am on day 1 and have until 11:59pm on day 2 to decide whether you want to keep the ticket or not.

-RM