Shakarris made this comment (and many others have, as well, in response to similar questions; it might just be one of the most often asked, yet hardest to answer for ourselves when it comes to actually making that hard decision) to your question: You asked when do you know when enough is enough.... you will just know.
Based on the facts that you are here seeking advice, you recognize the cycle of your wife's on-again-off-again drinking issues, and that you are asking the right questions...well, I'd say you are getting very near your answer. Be sure to listen to yourself, both your heart and your head. I like the advice about the intervention. It can't hurt to try...and that will surly help you get closer to knowing when 'enough' is reached for you and your boy, as stated so wonderfully by BlueJay6. Peace.