i think earlier is better. when i grew in soil, i'd transplant them as soon as i saw a handful of root nubbins fully emerged from the stem. the longer you wait, the more the roots will be adapted to your aero cloner environment and the more stressful a transplant to soil will be. that said, i once transplanted a pair of two-month old mothers from DWC to soil. one died and one made it, but after nearly dying and taking another full month to recover.
since soil is more forgiving, the sooner you get them in a rich soil enviornment with bennies, the better. the longer you leave them in the cloner, the more you're exposing them to risk of infection, system failure, or something else going wrong. aero might make roots grow like crazy, but if your ultimate goal is to put them in soil, why grow perfect, fuzzy aero roots when you're just going to damage most of them when you pack them in soil? i'd rather let them grow and get fuzzy in soil from early on so i won't have to disturb the roots.