If someone scammed me let's say, around a week ago, is it still valid to report?

How about for something that's months old?

As long as you have the evidence for it, it pretty much is.

Yup, like Rogue said, as long as you have evidence it's ok.

But you shouldn't do that often and wait for weeks or months to report because there's a high chance that you won't get reimbursed, considering the probability of the scammer disposing everything or even sink the toricredits.