If you pick it faithfully, you can have broccoli from now until the cold kills it in the fall/winter. If you live somewhere it never gets cold enough to kill it, you can have broccoli for a year if you do it right.

However, be aware that the offshoots produce heads much smaller than the big central head. Not that this is a bad thing, in some ways I almost prefer when mine become full of shoots later in the season and each head is about the size of a quarter. Convenient, actually, no trimming and cutting of a large head of broccoli in the kitchen, it comes pre-cut into bite sized pieces that way.

how far down should I cut do you think?

Don't count the ones that are growing right up against the crown. The ones that are on the stalk below it are the pairs I'm talking about.