M.L.
It sounds like a phrase she has picked up somewhere and is having fun saying. And why not? It gets attention, right?
If "too long" is in minutes, you can try setting a timer. "You can check this timer, and you'll see the numbers go down to one. When it gets to zero, we'll hear a noise, and it'll be time to go."
If it's "too long" in days, make your daughter a little calendar so she can put a sticker on each day that passes as she counts down to the big event.
Let her know that waiting is hard for everybody, and that when she can learn to wait a little better she's on her way to being a big girl.
After that, you can proceed with, "I've already told you the answer to that question. So do not ask me again." If she's just trying to be annoying, that's a different thing from being impatient. You know what to do.
Preschoolers think patience means sitting still for a count of twenty. That's about as long as they can really wrap their minds around.