© De Cecco
As far as quantities go, it really depends on the profile of the person (overweight, sporty, inactive, etc) and their appetite. All the same, the recommended average portion is 40g uncooked, which is 150g or 6 tablespoonfuls when cooked.
As for when to eat pasta, an inactive or overweight person can eat it at lunch or dinner depending on their requirements and what they feel like. If hunger persists, it's worth consuming some pasta both at lunch and dinner to avoid snack attacks.
Sporty people should preferably eat some starch at each meal, at least before they exercise so they've got enough energy, and again after their work-out to restore their glycogen levels.