You need something a.s.a.p.
You don't want to take much of someone's time.
There isn't a lot of time to do something.
You want to assure someone that a request is not problem for you.
You can see that someone is really busy.
You want to express gratitude for a favor. (2 expressions)
"I know this is last minute."
"No sweat."
"it's really urgent"
"you've got a lot on your plate"
"You're a lifesaver." / "I owe you one."
"I won't keep you any longer."