Excellent question. Resources in controlled environments can be "planned" because have good information about the outcome. How much input equals how much valuable output. There is no logical way to "plan" the right amount of resources for a strategic campaign. By definition, in an exploration of an unknown new territory, there are too many unknowns. As you say, we can only know the limit: we must only invest "excess" resources. We cannot invest so much we endanger our existing position by using the resources needed to support it.
But how much is the "right" amount of investment? The simple answer is that we cannot know beforehand. We can only make the decision on a day-to-day basis based totally upon what we discover.
This kind of question is why I always say that the purpose of learning strategy is developing a "gut feel" for front-line decision making. The problem is a holistic one. You can set certain rules ahead of time, for example, within three months we must see some progress that allows us to make a claim, but those rules are not hard and fast. Your gut may tell you to get out after two weeks. You must listen. Three months down the road, you may feel that putting in more time is worth it, even though you have no "facts" to base a claim on. But you make the decision based on what you have learned at that point in the process.
However, you must have the right viewpoint to make these gut decisions. A campaign is an exploration, not a commitment, a question being asked, not a prediction of a given outcome. Your future decisions must be based on what you find, not upon what you have hoped to find. This is where the whole idea of "planning" gets people into trouble. When people plan, they get emotionally invested in the outcome. For these people, the more they invest, the more committed they become to seeing the plan through. People throw good money after bad because they refuse to open their eyes to what they have discovered because they are too focused on what they had planned.