It depends who you're sending it out to, and what it's for.
If you're sending it out with review copies then you need at least 12-15 weeks.
If you're sending it out to potential readers (not that this is a good way to use a press release, but people do it) then a week or so before publication is fine.
There are lots of other variations.
Think what you want your press release to achieve. Consider what lead-time is required for that to happen.