Similar question here. I've never measured how long it usually takes to make the SCL rebuilds available to CentOS users, so I'm not sure if this long is to be expected as it came out in early/mid November.
I sent in a question to the mailing list a few days ago, but I messed up and did it before registering so it's stuck in a "waiting for approval" queue. It's not about the devtoolset-8 specifically, rather about the other new toolsets, but they do depend on some of the devtoolset-8's packages and will be developed at a different rate. Clang/LLVM-6.0 specifically depends on DTS 8, but Rust-1.29 can depend on DTS 8 if installed in full due to depending on Clang/LLVM-6.0 libs. Not trying to derail this thread, just noting the relation.