Unfortunately, there isn't anything really 'typical' about AP discounts. They've varied so much over the years. I remember 40% discounts; I remember 10% discounts.
It doesn't make much sense to book a reservation that you aren't prepared to use, at the rate you book it at. That's because if a discount rate becomes available, it is attached to specific inventory. So 'applying a discount' really means swapping some inventory around -- the point being that if there isn't already a comparable room available within the discount inventory, you wouldn't be able to apply the discount to your reservation anyway. So you have just as much of a chance applying the discount to your existing reservation as you have getting the discount without having an existing reservation. You're basically just letting Disney hold your deposit interest-free in the interim. So if you'd stay even if you don't get the AP discount, then go ahead and book, but otherwise, wait for the discount to be announced, and then book.