Ours vary. We've booked bounceback offers a year out, and we've done some last minute trips.
I guess I usually start a little over 180 days out. but I also have done plenty in under six months. Beyond 190 days out, I don't really do any planning- we just sit on the booking.
I someitmes tend to 'spring' when I see a great deal. It may be an airfare deal, it maybe a WDW deal. I don't book at full price.
It also matters who is coming with me. For big family trips, it is helpful to have extended time while everyone decides what they want to do, and we can pick dining for a large party.
Still, several times I've added a family member at the last minute! That's a true booking adventure!