It will be an unpredictably long wait until one is available. The speed of these things becoming available seems roughly proportional (inversely) to the popularity/sales of the tablet.
It's too early to tell how popular this will be, but I certainly haven't heard anything in regards to sales figure. This is not something that gets the attention of something like an iPad or even a flagship android tablet (Note 8.0?)
This is one of the disadvantages of fragmentation in the Android ecosystem. Too many different devices for developers to keep up with, especially the more obscure ones. But at least the Tab 3 8.0 has a physical presence in big retailers like Best Buy and Staples.
Just as a wild guess I'm guessing there will be a root guide by early August.