Also, these quests usually have 2 stages: first, talk to the settler about the quest, then actually do the quest. EACH of the stages gives you the same time frame in which to complete it. I.e. You have 2 days to talk to the settler about the kidnapping, and another 2 days to rescue the victim; or, 15 days to talk to the settler about ghouls problem or raiders trouble, and another 15 days to do it. But of course, you don't want to cut it too close and wait till the last minute to do either. Because the clock ticks constantly in these quests. You don't complete the quest unless you kill the boss enemy before the time is up, even if the boss is a few feet away from you.
Only the "Help Defend..." quests are the most urgent because of the 15-hour window; you can barely fast-travel once if you're far way. And I've even seen *two* of these quests active *simultaneously*. Adding to the pressure is that the "Help Defend..." quests are buried in the miscellaneous quests. Kidnapping quests have their own quest titles and give you a bit more time. The other quests that give you a 15-day window, you can sort of take your time a little. Also, the quests with a 15-day window will also give you a warning message 1 day before the deadline.