Here's a screenshot of me crossing the 10,000 follower mark. It happened while I was sleeping so I didn't get the exact moment. It happened about a week after I started trending. During the first 12 hour period, I got 2000 new followers and during the next 12, I got a similar amount. Then it started to decrease from there.
Here's a screen shot of my Tumblr email notifications, which came every 12 hours, in reverse order. The amount of followers steadily decreased as I went lower on the list of trending blogs on the Tumblr app. You can see that it suddenly drops from almost 200 to the teens near the top. I think this is when I stopped appearing on the app.
During this time, I also received this email from Tumblr saying on the was on their Radar. This meant I would appear on the side bar whenever people looked at their Tumblr dashboard. You can see an example below on the right. What pops up there is random though and even after hitting refresh a lot, I never saw myself appear. Maybe Tumblr is encoded so I don't appear for myself.
So I've trended on Tumblr and hopefully been on their radar. The next and highest step is to be on Tumblr's Spotlight where they recommend blogs to people looking for new things. It's the closest thing to a front page Tumblr has. While I could only see myself trending on the app, the Spotlight is easily accessible on their webpage. We'll see if I ever get on there.