https://steemit.com/bitcointalk/@candy49/how-to-qualify-for-signature-campaigns-on-bitcointalk
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In other words your activity level increases by 14 every two weeks, as long as you have made 14 posts in that two week period. If you have made less than 14 posts, it will increase by the number of posts, and if you have made more than 14 posts, it will increase just by 14 in the two week period.

It should take just over four weeks to graduate from newbie to jr member, another four weeks to make member, and another eight weeks on top of that to make Full Member. As you can see it is a slow process.
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とあるので、短期間に30投稿超えればいいわけではなさそうですね。