Memory is not kind to generals & political leaders with 90 day wonder plans. An iron clad certainty is that the only thing guaranteed is when violence is resorted to is when it is initiated.
While this is certainly true in general, it doesn't actually
help when it comes to the act of planning and executing warfare. The professional role of a general is to generate tactics, operational plans and strategic approaches which produce victory or avoid defeat, and since the ideal way for things to work is for imperatives to flow
downhill from the high level of grand strategy then the generals often don't have much of a choice.
Let's take McDowell, where he had a 90 day "wonder plan" as you put it.
The thing is, he really didn't. McDowell didn't go into the Bull Run campaign going "here is how we will win the war and it will only take ninety days" - what happened instead was that most of McDowell's forces had been mustered under the Militia Act of 1795. This act specifically stated:
That the militia employed in the service of the United States, shall receive the same pay and allowances, as the troops of the United States, And that no officer, non-commissioned officer or private of the militia shall be compelled to serve more than three months in any one year, nor more than in due rotation with every other able-bodied man of the same rank in the battalion to which be belongs.
The critical passage there is the point about three months - which, naturally, is about 90 days. The initial call-out took place on April 15 and there was a delay in getting the first formations organized, but at the time of the planning of the First Bull Run campaign (the fighting happened in the second half of July) large amounts of the new US army were about to expire through mustering out and the second volunteer call was only just beginning to muster in.
This meant that there was a major dilemma stemming ultimately from the politics. Either the "75,000 volunteers" army was ready to fight now after three months of training (which McDowell thought was
not the case) or it took more than three months to train volunteers up to the point they could fight - which would mean the Union could take
no major offensive action until October (which was getting towards being too late for campaigning anyway).
There was thus a political pressure towards a battle within 90 days of the callout, and indeed Lincoln made it a political imperative - that's where "All Green Alike" comes from, he was trying to pressure McDowell that while McDowell's forces weren't ready to fight nor were the Confederates ("You are green, it is true, but they are green also; you are all green alike.")
McDowell's judgement was not to fight, but he was persuaded to by pressure from Lincoln.
Now consider what it looks like if the Union doesn't fight for the whole summer...