I started mine about 3 months after I joined Medium. I'm going into month 5 now. Once I felt like I had connections that would join and help spread the pub, I felt ready.

Really though, as long as the publication has a good topic, there really isn't anything that will keep it from growing as long you promote it. I've seen people start pubs when they got to 1,000 followers. I've seen people start pubs when they had under 100. Both ended up successful. There really isn't a best time as long as you're willing to promote and put the work in that's needed to grow it.