most plants will start to show a nitrogen deficiency wich is yellowing of the lower older leaves on the plant wich is a good indication that you need to start using nutrients starting at a reduced strength.
while there is alot of deficiencys that could occur that is the most common one that tells you when to start feeding you might look at the link below for og faq wich has more on nute deficiencys.
Drugs-Forum - Cannabis cultivation: Overgrow's GrowFAQ