If you aren't going to transplant the extras (which is a sad waste) and plan to just snip all the extras off, then the sooner you do it the better. There is nothing to be gained by letting them grow on and they are only stressing the ones you plan to keep.

Dave

I started my broccoli, kale, brussels sprouts and lettuce the same way but I want to keep them, is it better to separate early as opposed to later due to the roots binding together? I separated my kale yesterday, They sprouted about 4-5 days ago and they were still standing this morning, any comments, experiences??

I am just worried that I will mess up if I try to transplant when I thin and lose all my seedlings.. I have never done this before but if I were to try to transplant them, when do I do this?

My experience is to thin them sooner rather than later, I've had good luck for a few years doing it. All my Kale that has been transplanted and survived but lost a few due to overseeding, it happens. I use a spoon and slide it an inch away parallel to the tap root and dig 2" past the length of the plant itself, it should lift out with a little help of your finger, have the hole in the new pot ready thats deeper than the lenght of the root. Then just place the seedling in its hole and slide the spoon out. Trial and error will teach a lot but a hint cant hurt. I would do this when the plant is like 3-4" tall, or when it has set its true leaves. The roots will bind quickly so its better sooner than later, I separated mine when they were just sprouting, about 2"-3" tall, I'd say its safe then for the broccoli.

Thanks for the advice greenthumb..I think I will try a couple tomorrow and see how it goes..