When asking an unknown researcher about their work (and, most specifically, the data they used and the availability of it) via e-mail, what timeframe could one expect as a "normal" one in which an answer would be expected? If you don't get an answer within this timeframe, how one tactfully go about persisting, if at all?

This is, of course, after formulating the e-mail in a way which describes you and the particular project you're doing that the researcher (hopefully) takes interest in it.