Objection 1. It would seem that a habit can be caused by one act. For demonstration is an act of reason. But science, which is the habit of one conclusion, is caused by one demonstration. Therefore habit can be caused by one act.
Objection 2. Further, as acts happen to increase by multiplication so do they happen to increase by intensity. But a habit is caused by multiplication of acts. Therefore also if an act be very intense, it can be the generating cause of a habit.
Objection 3. Further, health and sickness are habits. But it happens that a man is healed or becomes ill, by one act. Therefore one act can cause a habit.
On the contrary, The Philosopher (Ethic. i, 7): "As neither does one swallow nor one day make spring: so neither does one day nor a short time make a man blessed and happy." But "happiness is an operation in respect of a habit of perfect virtue" (Ethic. i, 7,10,13). Therefore a habit of virtue, and for the same reason, other habits, is not caused by one act.
I answer that, As we have said already (2), habit is caused by act, because a passive power is moved by an active principle. But in order that some quality be caused in that which is passive the active principle must entirely overcome the passive. Whence we see that because fire cannot at once overcome the combustible, it does not enkindle at once; but it gradually expels contrary dispositions, so that by overcoming it entirely, it may impress its likeness on it. Now it is clear that the active principle which is reason, cannot entirely overcome the appetitive power in one act: because the appetitive power is inclined variously, and to many things; while the reason judges in a single act, what should be willed in regard to various aspects and circumstances. Wherefore the appetitive power is not thereby entirely overcome, so as to be inclined like nature to the same thing, in the majority of cases; which inclination belongs to the habit of virtue. Therefore a habit of virtue cannot be caused by one act, but only by many.
But in the apprehensive powers, we must observe that there are two passive principles: one is the "possible" [See I, 79, 2 ad 2] intellect itself; the other is the intellect which Aristotle (De Anima iii, text. 20) calls "passive," and is the "particular reason," that is the cogitative power, with memory and imagination. With regard then to the former passive principle, it is possible for a certain active principle to entirely overcome, by one act, the power of its passive principle: thus one self-evident proposition convinces the intellect, so that it gives a firm assent to the conclusion, but a probable proposition cannot do this. Wherefore a habit of opinion needs to be caused by many acts of the reason, even on the part of the "possible" intellect: whereas a habit of science can be caused by a single act of the reason, so far as the "possible" intellect is concerned. But with regard to the lower apprehensive powers, the same acts need to be repeated many times for anything to be firmly impressed on the memory. And so the Philosopher says (De Memor. et Remin. 1) that "meditation strengthens memory." Bodily habits, however, can be caused by one act, if the active principle is of great power: sometimes, for instance, a strong dose of medicine restores health at once.
Hence the solutions to the objections are clear.
Source: Summa Theologica (New Advent)