Maybe never . . .
See Antonio J, Uelmen J, Rodriguez R, Earnest C. The effects of Tribulus terrestris on body composition and exercise performance in resistance-trained males. Int J Sport Nutr Exerc Metab. 2000 Jun;10(2):208-15.
"In one of the few well-controlled studies to examine the effects of tribulus terrestris on body composition and exercise performance looked at 15 resistance-trained males. Subjects received either a placebo or a large dose of tribulus (1.5mg per pound of body weight per day for 2 months). Results showed no changes in body weight, percentage fat, total muscle mass or muscle strength related to tribulus supplementation."