Currently ADRs can be made 90 days in advance, however the booking window is changing to 180-days out on 10/27. So, if you are staying at a WDW resort your booking window opens 180 days from the day you check in. At that 180-day mark you'll be able to make ADRs for up to 10 days. This is known as 180+10.
If you are staying off-property your window opens up 180 days from the day you want the ADR. You will only be able to book for that day and will have to call back (or log on), day by day, for additional ADRs. Another option is to wait until 180 days out from your last day and make all of your ADRs with one call (or via one internet session) on that day, however you risk losing out on some of the more popular places. There's hope that the return to the 180-day window will alleviate some of that difficulty, but it is up to you whether you want to take the risk or not.