When to transplant?

I have been growing from seed - Currently on day 17 in tray with humidity dome & heat pad. All are doing well, good root development, they are between 2 & 4 inches and 4 of them are pushing on the top of the dome.

I am wondering how long I should wait to put transplant them into my DWC system - Is there a rule, I can see 5 to 6 roots coming from the bottom of the rockwell cubes.

Thank you for your help -

Go for it, they are ready to go