When to go from 18/6 to 12/12?

[smoknjoe]

When you get alternating nodes...

Alternating nodes is a sign that the plant is mature, but doesn't mean you have to flower. You can keep going at 18/6 for as long as you like (space permitting) keeping in mind the plant will double in size after switching to 12/12.

To answer your question more directly, you can switch to 12/12 whenever you want, as long as its not still a baby.