What are the guidelines behind assigning level to a ritual?
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:
- Nail down whether you want the ritual to be commonly used in Heroic, Paragon or Epic-tier situations; that will give you an initial 10-level range to work with.
- Examine a few existing rituals for some levels in the middle of the chosen tier and decide whether they sound about equal, higher, or lower in power and effect than your prospective new ritual. That gets you to a 3-4 level spread.
- Within that spread, the exact ritual level probably doesn't matter terribly much. But I'd repeat the previous step in greater detail to get a sense of the relative effects of other rituals and make a commitment accordingly.
- Is your campaign pretty high-magic? Maybe err on the lower side if you're stuck waffling between two levels. If your campaign is more low-magic, round up instead to make the ritual harder to learn.