anywhere from 3rd to 8th depending on the strain, stretch, space, light
im doing mostly indica pole plants right now, tried to FIM a couple, they hated it. the haze i have next to them i did at the 6th or 7th and she loved it...
but i mean i guess to answer the "minimum" height, i'd say about 6-8 inches, or technically whenever a true growth starts forming from the seedling stage. may not be wise, but you can do it with reaction and survival from the plant...