What are the guidelines behind assigning level to a ritual?
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I don't believe there are any official guidelines.
First off, do a thorough scan of existing rituals and see if there isn't one already in existence in the published products that matches the intent of the ritual you mean to create - is this really a new spell or just a reskinned or slightly unusual case of another ritual's function?
If you do decide there's some important and hitherto nonexistent effect you want to have as a ritual for your campaign, I don't believe there is any hard-and-fast formula for this; there's more art than science to assigning levels. Here's what I would do: