So when do you INTJs experience it? Here's a description (from Wikipedia):
Flow is the mental state of operation in which a person in an activity is fully immersed in a feeling of energized focus, full involvement, and success in the process of the activity.
Ten factors accompanying an experience of flow:
1. Clear goals (expectations and rules are discernible and goals are attainable and align appropriately with one's skill set and abilities). Moreover, the challenge level and skill level should both be high.
2. Concentrating, a high degree of concentration on a limited field of attention (a person engaged in the activity will have the opportunity to focus and to delve deeply into it).
3. A loss of the feeling of self-consciousness, the merging of action and awareness.
4. Distorted sense of time, one's subjective experience of time is altered.
5. Direct and immediate feedback (successes and failures in the course of the activity are apparent, so that behavior can be adjusted as needed).
6. Balance between ability level and challenge (the activity is neither too easy nor too difficult).
7. A sense of personal control over the situation or activity.
8. The activity is intrinsically rewarding, so there is an effortlessness of action.
9. A lack of awareness of bodily needs (to the extent that one can reach a point of great hunger or fatigue without realizing it)
10. Absorption into the activity, narrowing of the focus of awareness down to the activity itself, action awareness merging.
Not all are needed for flow to be experienced.