It's sinful when you drink enough of it that it effects your judgement. If it causes small errors in your judgement, it is venially sinful. If it causes graves errors in judgement, it is mortally sinful. If you drink frequently enough that it causes minor health issues, it is venially sinful. Major health problems, which you are aware of while you are doing it, and it's mortally sinful.
If none of the above, it's not sinful at all. However, if you have a weakness with regards to it, then it demonstrates a lack in the virtue of moderation. It is therefore a vice, which in itself is not sinful, but by definition, can easily lead you into sin.