Reinforcements are always delayed for the enemy. Usually, the battle is already finished before they arrive. (For reference, I play on VH/VH).

I've seen some delayed enemy reinforcements that actually arrived. But the enemy is already routing.

1) There's probably a fixed time in which they are cued-in to arrive (when delayed), but the battle is normally over before that time arrives.

2) There is probably a special ratio that when exceeded, the enemy gets a delayed reinforcement.

I've fought a ~3000 vs ~3000 siege battle (I was the attacker) and the enemy reinforcement arrived (~400). They were delayed though, but they arrived.