The game expects the reader to intuit that the spell windy escape neither actually makes the caster briefly insubstantial nor provides DR 10/magic forevermore for the price of a 1st-level spell, but, instead, that the spell windy escape grants its caster DR 10/magic only against the one incoming attack that's being made against the caster right now. Future attacks are unaffected. (There's no reason, for instance, for a spell to have Casting Time: 1 immediate action if the spell's effect grants the caster a super power that lasts the rest of the caster's life! I mean, why not just cast such a spell when it was first learned?)
A Duration entry of Duration: 1 enemy's attack perhaps would've been clearer—or, at least, more accurate—than the windy escape spell's Duration: Instantaneous. Most of the time when a spell has a duration of instantaneous, after the spell's cast, the spell's effect persists forevermore. For example, casting the spell fabricate turns raw materials into finished goods forevermore, and the spell wall of iron conjures forth an iron wall that remains forevermore.
By the same token, casting the windy escape spell should forevermore grant the caster the ability to "respond to an attack by briefly becoming vaporous and insubstantial, allowing the attack to pass harmlessly through" the caster. Unfortunately, the actual effect of the windy escape spell isn't clear from the spell's description, the description combining fluff and crunch into a confusing whole. When the spell's description says, "You respond to an attack by briefly becoming vaporous and insubstantial, allowing the attack to pass harmlessly through you" (emphasis mine), the spell's describing the circumstances in which the spell can be cast, not the spell's effect. The spell's actual effect is described in the next sentence: "You gain DR 10/magic against this attack and are immune to any poison, sneak attacks, or critical hit effect from that attack" (emphasis mine).
The DR granted by the windy escape spell functions normally with some exceptions. The DR reduces the damage dealt by that lone attack by 10 points if the attack is made with a mundane weapon (like a typical longsword) or a typical natural weapon attack (like a giraffe's slam) but does not reduce the damage dealt if the attack deals energy damage or if the attack is magical in nature or if the weapon used is a magical weapon (like a +1 longsword or a giraffe's slam affected by the spell magic fang).
However, even if the spell's DR is bypassed, the windy escape spell's caster remains immune to "any poison, sneak attacks, or critical hit effect from that attack," making the spell useful even if the the enemy rogue attacks the caster with her magical rapier, her sneak attack damage negated for that attack, the poison dripping from the blade briefly not a concern, and the possibility of that attack becoming a critical hit absent.