Posted: 10/04/2017 at 15:57
I find I have more success with onion sets if I start them off in cells in a cold frame. That way it is easy to keep an eye on them and check their root growth. The shoots will appear before roots and it is much easier to discard any which aren't up to much.
Don't know your location but if your onions have shoots of the height you mention, you should have roots forming by now.
As pointed out above, you have a good few months to go yet so unless they are completely soft ( as in rotten) you should be ok.