Pfahlwurzel

A taproot can be visible within a day or so of germination. So, it's not always the 'best' way to trigger potting up. How long between current irrigations? If it's still several days, you know it's not drinking fast enough to warrant a larger volume. Rate of drinking is the key. You can always pot up, so it's not a law written in stone. As long as the next pot size isn't too large, it won't make much of a difference. What you don't want is a plant sitting in a wet, stagnating medium after potting up that the plant's roots can't make use of in a timely manner. Avoid that and you did it "right."