Well we have not sown ours yet as it is too soon. From when you sow them it will take several weeks before there is any visible top growth, the seed is busy putting down roots.

If you are asking how long it takes from sowing seed to harvesting then you can start using the thinnings once they are little finger size (possibly 3 months) and then just keep on using. You should expect to have harvested all the crop for storage by early November at latest aas they can't cope with frost.