I'm really not sure what you're looking for here. Everyone is going to have a different standard on this one, and come to their own conclusion. Hopefully, they're rational and flexible enough to change their mind when presented with new information.
If you're wondering about the specifics on why I think the current prohibition on cannabis is unjust, you can start in this thread.
- More arrests for simple possession in the US than all violent crimes combined (a number that's upward of 800 000 per year). Those arrested are often affected in one way or another for the rest of their lives (whether they go to prison or not). I'd argue that that penalty is especially extreme since the crime, as far as I can tell, isn't unduly harming anyone other than (potentially) the user himself. Id someone does something that harms someone else, punishment may be reasonable. Otherwise, it's unjust.
- In decriminalized states, there has been little evidence that decriminalization leads to an increase in use. Further, with resources devoted to other methods of combating addiction have proven a boon to many societies (harm reduction strategies have been both more effective and cheaper).