Ditto what Kalen2 said. As soon as the seed cracks and the root emerges - you know the seed is viable. Plant it right away. The longer the taproot, the greater the risk of it being damaged and/or not surviving planting. Good luck!
ive grown some directly after the taproot emerges and some up to two weeks after. it really doesnt matter, though i do agree with pablo that they are more susceptible to failure the longer the taproot.