How frequently do most people look at their camera roll? I'd be surprised if it's more than a few times a week on average.
Does an attacker even need access to the phone? If iCloud is syncing your photos, your phone will eventually see all your pictures. Unless I've misunderstood how this works, the attacker only needs access to your iCloud account.
For me it's probably 5-7 times per day, but I also take a lot of photos.
I think a few times a week is probably low-balling it, even for an average.
If you hack my phone and plant some photos with a sufficiently old timestamp I'd never notice them. I can't imagine my situation is all that uncommon either.