As jm94 said, Rodinal concentrate lasts for decades, even in partially-used bottles.
Discoloration, even if severe, does not affect its performance.
Once mixed, it should be used within about 15 minutes. I always mix it while the film is in the pre-soak.
I've been using Rodinal for over 50 years (not the same bottle) and have never experienced any variation in performance with age.