Don't think about it too much and just spend enough to keep your CI moving up. Or, once it's reached 100, spend enough to maintain it.
As you get larger you'll notice your CI growth slows, stays flat, or even starts to decline. That's your clue to spend more money. Just add another marketing plan. You can have up to 5, so there's no need to necessarily cancel one, just add another one as you notice your CI isn't going up that fast, and keep slowly adding more (either more plans or bigger plans) to keep your CI moving up at a good pace.
And just because you start flying internationally doesn't mean you HAVE to have an Int'l marketing plan. I've got a few dozen international routes without any International marketing. I just run 3 Nationwide and 2 States -- that's what it takes for me to maintain a CI of 100 with the number of routes and passengers I've got (#1 for pax in JA#1@ approx. 1.5M/week). At some point I imagine I'll have to turn one of those States into another Nationwide.
In my experience adding a second plan is more cost-effective than jumping up to the next largest plan. I only begin upgrading once I've got at least 3 plans of one level going already.
So I'll start with 1 "Municipality/State". Then add another. And another. And get to at least 3 of those. Then I may add another or upgrade one of those 3 to a "Country". And keep upgrading those. And maybe get up to Continent.
Maybe the difference between "Country" and "Continent" has an appreciable effect someplace like Europe where it's common to fly to hundreds of locations that are out of your country. But I've noticed no such effect in North America or any problem on my International routes despite having no international marketing. All that matters is that my CI keeps moving up.