I'd agree with mashup here. I don't see any reason for why not to use "seit." As opposed to in English, in German it doesn't matter whether we're talking about a specific point in time or a period of time. In both cases, we can use "seit." Therefore, it can mean both "for" and "since". The sentence "Ich habe ihn eine Woche lang nicht gesehen", which Georg suggests, also expresses the fact that the speaker hasn't seen a specific person for a week; I see a slight difference in meaning, though. And there there definitely is a stylistic difference.

"I haven't seen him for a week." = "Ich habe ihn seit einer Woche nicht mehr gesehen."