I don't owl, because I can't. Well, I guess I can, but it doesn't sound like an owl when I try.
Anyway, to answer your question, IMO, it's never too early. It's a natural sound. The birds might, or might not, respond, but it's not going to hurt anything.
I try to wait until I hear a real crow before using a crow call as a locator, but you can hear a real owl anytime day or night.
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