Regular
DAS Return Times obtained in the park will not run out. It is based on the standby wait time. So as long as the standby queue is open you should be able to get a DAS Return Time. You and your whole party will need to have entered the park to get a Return Time on the app; it does not open early like Genie+.
Regular DAS Return Times obtained in the park do not expire. You may use it anytime after the stated time. So a 2pm return time you could use at 5pm. However, you cannot get another DAS Return Time until you use it.
DAS Advanced Selections scheduled on the chat do expire. It will be good for at least a 1-hour window. Some folks have reported it disappearing shortly after that 1 hour, others have been able to use it a little later. I would plan to use it within the hour. You can also get a regular DAS Return Time in the park for the same attraction.