A: Warum ... nicht solange ...?
B: Das lass ich mir nicht zweimal sagen! Ich bin schon gespannt, was ...
I assume this is an enthusiastic way to express your agreement with a suggestion, but I wonder if it is the equivalent of "Now you’re talking" or "You can say that again".
If so, it is interesting to note that in English you ask for the statement to be repeated, while in German saying it once suffices.
In my opinion, "Now you're talking" or "You can say that again" do not have the same meaning as "Das lass ich mir nicht zweimal sagen".
The expression "Das lass ich mir nicht zweimal sagen" directly translates to "You don't need to tell me that twice", which sounds kind of odd. Another translation could be "I won't miss that for sure".
In your example, "Das lass ich mir nicht zweimal sagen" means that this person cannot wait for something ("Ich bin schon gespannt, ...) and he/she sure does not want to miss what happens then (whatever that is).
I hope this helps you.
It is the easiest, to view the context. People tend to postpone unpleasant tasks, therefore may need frequent reminders. Example:
Wie oft muss ich dir noch sagen, dass du dein Zimmer aufräumen sollst? (How often do I have to repeat, that you shall clean up yoour room?)
So the task must be quite a pleasant one, if no reminder is necessary at all. A typical use is:
Nimm doch das letzte Stück Kuchen!
So the response
Dass lasse ich mir nicht zweimal sagen!
indicates something like it is a pleasure to comply, typically even exceeding that, i. e. that you wanted to eat the cake anyway, but were too shy to ask for it. So I agree with Devon, that your example is not the typical one, which is a response to a request.
You can use this expression when anyone suggest you sth. and you’re sure to take the opportunity.
Another German phrase would be:
Diese Chance lasse ich mir nicht entgehen!
You could translate it to “I don't have to be told twice”, but “Now you’re talking” is also good. An equivalent to “You can say that again” would be “Das kannst du laut sagen!”
I'd use: Das lass ich mir nicht zweimal sagen! in the following context.
Chef: Okay, heute ist nix los hier. Du kannst heim gehen.
Ich: Das lass ich mir nicht zweimal sagen!
The counter-phrase to this, when somebody needs to be told something twice or more is:
"Brauchst du noch eine extra Einladung?"
As @guidot said, the sample with the »Kuchen« is a typical one. However, there's another use for your expression.
Heinz hat mich schon wieder »dicker Zwerg mit langem Haar« genannt, aber jetzt reichts, das lasse ich mir [von dem] nicht zweimal sagen!
(Heinz called me again »fat dwarf with long hair«, but now I've had enough, he can't say this twice!)
The speaker is expressing that he won't accept the repeated insult; he could swallow the first one, but can't stand the duplicate. He seems to intend taking action against Heinz.
Alternatively he could have said:
... jetzt reichts, einmal lasse ich mir das [von ihm] sagen, aber nicht zweimal!
... jetzt reichts, das sagt [d]er mir kein zweites Mal!
... jetzt reichts, das hör ich mir nicht zweimal an!
