As the GM in question, I'd like to provide a bit more context on this.
The background is a PC using a biomancy ritual to "power up" for a big fight at a known time and location - enhancing his senses, instinctual responses, and physical acuity. His original statting of effect was "substitute a 12 for any alertness, stealth, investigation or athletics check, up to six times before the spell expires" - which he said was priced as a complexity 12 effect, plus five to get it usable six times instead of one. (plus a token bit of extra complexity to get the overall duration up to long enough to cover from end of ritual casting to end of known fight).
Now, it's fairly clear from the rules that you can make a ritual at complexity, for example, 12, to substitute that complexity for a single specific skill check - for example, something that lets you pull off a cheetah-like burst of speed, substituting for athletics. And you could even give that effect a duration (as long as it's not an attack), at one additional complexity per extra exchange. (Or possibly longer, if - like stealth - one skill check normally covers more than a single exchange.) But I don't think the rules even imply you can use the same cost structure for a spell that would let you sprint one exchange, not use it at all for the next three, boost your alertness the exchange after that, and then run it as a standard block against attacks for another four exchanges.
My suggested implementation was to use the ritual as a stack of navel-gazing maneuvers, adding temporary aspects like "eyes of the eagle", "speed of the panther", etc, with one free tag each, and the aspects lasting the duration of the ritual (figuring that the base duration for such a ritual is "a scene", and four shifts will push that far enough up the time chart to last "until the next dawn").