This is my fault. I thought I could condense all of the possibilities into one sentence, but that isn't the case. Now that I see what Izmael is saying. I like trying to simplify into a single equation if possible
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These are the three possibilities the top spell slot gives you, based on the spell:
1. If the casting time exceeds the cooldown time, you can save up to the cooldown time.
2. If the cooldown time greatly exceeds the casting time, you can save up to the casting time.
3. If the cooldown time is greater than the casting time (but not by a large margin), then you can save the cooldown time minus the additional time it took to finish casting the spell after the cooldown has finished.
EDIT: I think this is why I said it "saves the casting time" earlier, because two out of the three possibilities listed are times shorter than the cooldown time. So if I had to give a super short answer, technically "saving the casting time" (or close to the casting time) would be the more common scenario for the majority of spells. At least from my experience with caster classes. I certainly haven't played all of them, so I may be underestimating how many spells fit scenario 1. But ideally the discussion should be on a per spell basis for maximum clarity.