Gotu kola or Vallarai (Centella asiatica) has been used to treat many conditions for thousands of years in India, China, and Indonesia. It was used to heal wounds, improve mental clarity, and treat skin conditions such as leprosy and psoriasis.
Most of the human studies (on Chronic Venous Insufficiency) on this herb have used a centella asiatica supplement two to three times a day, and at each dose the total saponin dose (asiatic acid, madecassic acid, asiaticoside, and madecassoside) has totalled 30-60 mg given a total daily range of 60-180 mg total saponins.
While there are currently no human studies on cognitive enhancement, rat studies have noted success with 200-300 mg per kilogram of the overall plant extract (since the saponins may not be the only active ingredient for cognition); this suggests a human dose of 32-48 mg/kg and thus:
2,100-3,300 mg for a 150lb person
2,900-4,400 mg for a 200lb person
3,600-5,500 mg for a 250lb person
The above dosages ranges are but estimates for cognitive enhancement. Currently, 500mg of centella asiatica twice daily has shown anxiety reducing effects in humans and 750mg of a 5% asiaticoside extract has enhanced mood state; while these doses are active on the cognition, it is not yet demonstrated if they are the dose needed to boost learning.