Hi! Good question. It's more of an art than a science, stalking your trip. Fare sales can happen at anytime. That said, I think starting your stalk 3 months out should give you a good window of time to get the best trip. Remember, that when the airlines put seats on sale they often do just a few seats in that fare class, so it is important to know when to strike. Keep an eye on the price, most of the time you should notice that the fare will stay pretty consistent with some minor fluctuations. When you notice a dip, it's time to grab it! The best thing to do is to see what the price is currently coming up at and then target your notify price about 20% below that. This should trigger the fare alert so when the fare drops you can assess if it's a good enough price for you. Remember that the fare alert can be adjusted at any time by adjusting your notify price.
Good luck and happy stalking!