the moment its leaves are emerged. can't photosynthesize before then...
you still want lights on before -- so they don't stretch when they first sprout. LED lighst normally ~24" are better at 30-36" for a seedling. any stretch, move it closer... any stacking of nodes with no internodal space, move it further away. if it falls over, move it away, and just prop it up, it'll be fine.
light is generally bad for roots. so if soaking longer, keep it dark. if planting, if it is burried a short bit, the seed should be pulled off by the time it emerges, but not always. wait for leaves to crack it open to the point you simply have to gently tap it off... don't try to pry it off. you'll potentially damage a very sensitive plant at this particular moment in time.
some say wait for an inch or more... i say get it in soil immediately, and i wouldn't soak it to the point it germinates. maybe, just till it sinks, then plant it. too many risks... touching an exposed root.. root in light.. etc etc.