It kind of depends on what the Omen Rings actually do? We can probably assume they're some sort of power up. But, we don't actually see Sommers or Killingham get any stronger after getting them. Maybe we will next chapter, but not yet.
The Omen Ring does seem to heal them. So, maybe it's a purely healing ability? Like Goku throwing a Senzu Bean to all his friends in Dragon Ball? Healing "onion rings"? If that's the case, it's possible that Imu gives his God's Knights a "Omen" every time they return from a big battle to help heal them up. It could potentially be a pretty common thing Imu does for them all the time. (Great...now I kind of want to see Luffy try to eat one of these Omen Rings, just to see what would happen.)
But, assuming it is some form of power up, and assuming it's not just something they've only ever experienced before in training...I don't know. Imu didn't seem to use them in God Valley, which was probably the biggest battle we know about. It's possible there's some other big battle that we've never heard of before that will come up later, where Imu used the Omen Rings on them.
But, the only other time that I can think of that could MAYBE be when Imu could have used these Omen Rings was maybe the God Knights trying to get Shanks back after he stole the Gum-Gum Fruit? I feel like a battle with Shanks might be the only other time I can think of when Imu would go all out like that. But, that feels like a stretch. I'm not sure if Imu would consider Shanks that big of a threat. Given how much Imu "cares" about his God's Knights though, it's possible that Imu took Shanks's betrayal pretty personally, though.
Maybe some battle with the Revolutionaries? But, we've never heard the Revolutionaries talk about the God's Knights before. So, who knows?
So, I guess I'd say some battle with Shanks, was probably the most likely last time Imu used these Omen Rings. If they aren't just a healing item. And if they weren't used in some big fight we've never seen. The Omen Rings look cool, but we really don't know what they're all about, yet.