Decay allows Tomura to destroy anything his hands touch, making it crumble down to bits. Whether the target is organic or not, the Quirk will work without fail. The speed of the disintegration can vary from slowly to almost instantly.
The decay will only occur when all five fingers touch the target, indicating that the activation of the Quirk is not voluntary, or at least that Tomura has clumsy control over his own Quirk, avoiding using all his fingers to touch objects as a safeguard. As such, Tomura is often careful with his hands when touching objects he doesn't want to destroy.