short answer is at week 3-4
in my experience they are usually ready between week 2 and 4 depending on how hot your soil is and how big of a pot your started out with, a "weak" seedling soil in a small Cup might only keep enough to feed em for the fist two weeks, a hot one can maybe last 4-5 week`s, or organic super soil (if mixed correct) can last all grow, in coco who keep no nutrients I start on 1/4 strengths right away in a soil I start it on week 2-4 but agin on 1/4 strength to start out with as small seedling don't need much compared to big plants, specially in a soil that maybe still keep a bit of nuts, I rarely hit Max before flowering, specially with a short Veg time, if I Veg for longer (then 6 weeks) I might, but with a repot or two along the way to flowering I don't as the soil keep enough ..
remember the time where it need the most (N.) is in the fist two weeks of flowering and the most P. at week 4-7 of flowering, where big flowering plants in hugh pot`s demand a lot more then a small Veg plant
but in the end its all about reading your plants, need to do a few run`s to find out, also different strings have different needs, so its not that easy to say, a fast growing plant under strong light also use a lot more then a slow growing plant under a single CFL.
if your bottom leaf`s begin to lighting up and get light greenish/yellow it mostly a sure sign that its time to feed (that or your PH is of or it need Ca/mg to be able to take up the nuts)