How long will you be in jail for smoking weed?

Depends on the country, state and the amount.

In almost every western country, simple first time marijuana possession is rarely enforced with more than merely a fine. Repeated violations can lead to imprisonment, at least in the United States. There are a lot of other side effects from having a marijuana possession on your record, among them, I've heard, is that you can no longer get a student loan or some such.