Well, well before. The earlier you can, the better. Journalists are tough to get in touch with and cold outreach is often lost in a sea of similar requests.

The ideal situation is to identify mutual contacts (facebook or linkedin) cosy up with an introduction and be active for weeks to month in their life. Comment on their personal blog, retweet their posts, etc. By the time you launch your campaign, you're a friend they want to help out, not email #135 of that day.

Hope this helps!