Why is an put here?Yes.
Not in current usage, no. I hadn't noticed till now, but of course in other uses you have to spend a period of time doing something. In this expression, the thing you're discussing is implied: it was something amazing.I think there's a variety difference here because in British English the sentence is not correct as it is. As emsr2d2 shows in the examples in post #5, you have to complement the sentence with a preposition phrase or an -ing phrase.
Is that not true in American English?
Why is an put here?
I have just spent an amazing few weeks.
I'd expect to hear all about it in the following sentences.Doing what?