Ruling on rituals?

I seem to recall that one sort of ritual, done to give a bonus to something else (like the arming of orlanth done to strengthen hero questers) takes one hour per +1 bonus to be given. That is pretty straightforward.

But what about "rituals" more broadly. For example, in Gathering Thunder, it is mentioned that the one Barntari king has a ritual that defines an area as Barntar's field, and essentially lets "weeding" magic be used against the enemy in it. What would the difficulty be of something like that? How long would it take?

Yes, of course this can be narrated dramatically, but it helps to have a consistent view to start with. Something like "most rituals take about an hour and face a default resistance of 14" Or "most rituals take about as long to perform as their bonus will last, and have a resistance of 14 plus a range modifier or multiple target modifier appropriate to the effect."

I know this isn't in the rules, I'm looking for people's gut feel (or reasoned analysis for that matter) of what makes sense.

--Bryan