A flare isn't really something that's quantifiable or easily defined. Any sudden return or worsening of symptoms can be considered a flare. And you can have particularly bad flares where your symptoms get absolutely terrible out of nowhere or you can have mild flares where your symptoms just get a little bit worse. But, in my experience they usually come on out of nowhere. And when you do get them, most doctors will prescribe steroids to help quickly reduce the major symptoms. I mean, you might be able to tell that one is coming if you start to get bad cramps or if you see a return of some of your telltale symptoms.
Crohn's is totally different for just about everyone... Everyone has different symptoms and it effects everyone's body differently. In my case, I don't get diarrhea or have to spend hours on the toilet like a lot of people do... I just get nausea and really bad cramping. A lot of Crohn's is trial and error, figuring out what works best for you... Learning what foods cause you to flare or agitate your illness and learning what lifestyle changes need to be made so you can cope and most importantly, finding a medication regiment that controls your disease. Most of this can be learned with help from doctors and dietitians, but a lot of it you kind of have to figure out on your own, since Crohn's is so individualized.