As a general rule, you'll often notice your plant's leaves telling you when it's time for more doses of fertilizer. Signs such as:
1.) The leaves start to turn a lighter-green than they usually are. When the green starts to progress to yellow, you know it's time to up the amount the plant feeds. I start with 1/8 teaspoon/gallon, then when the plant needs, 1/2 per gallon, then 2/3, then finally 1 teaspoon per gallon, after the plant has grown quite large and has grown extensive roots.
2.) The plant's leave petioles often start to "droop" as if the plant is slightly unhappy. They'll develop a dark purplish color, instead of a more-greenish color.
There could be other signs that a plant is ready for more nutrients, but I noticed those are the quickest to spot, at least with my plants so far.