Don't wondrous items have standard action to use? Spell Completion and Spell Trigger items, on the other hand, specify that if the spell has a longer casting time, use that instead. That's because using those items is the equivalent of casting a spell, whereas wondrous items aren't. They just do it. So I would say that since crystal balls are wondrous items they require a standard action to use. If, on the other hand, you had a staff with scrying, it would take 1 hour. Remember that wondrous items don't follow spells verbatum. They simply have effects which are similar to spells. The crystal ball says that it function like scrying, but greater scrying says that in its description as well. Also, crystal balls break the normal rules for scrying since they can't benefit from other spells (such as see invisibility), not even with a % chance, unless it's enchanted with those spells (in which case it always works).