When you buy your points with DVC, you are only guaranteed to be able to use your points at some point during the year.
However, where and when and for what types of room is all based on availability. So, the sooner you call, the easier it will be to secure exactly what you want for your trip.
Those owners who take advantage of booking during the "home resort" booking period, from 11 months up to 7 months in advance, typically get what they want.
However, if you want until the 7 month mark or sooner, then you are competing with all DVC owners for rooms at all the resorts and it becomes harder to book. Last minutes trips, like 3 to 4 months out, are even harder to put together. DVC really works well for those that can and do book trips far in advance.
Of course, as PP mentioned, there are times and rooms that are very difficult to get, even when you call right at 9:00 am est, at 11 months. DVC popular times are not necessarily the popular times for the parks. Early December is a very busy DVC time and if you want to stay then, you need to book early.
Getting AKV concierge is tough as well as BWV and SV rooms at BWV. So are SV rooms at BLT. Nothing is impossible and some have luck even scoring these at times when things are busy, but the longer you wait to book your trips, the harder it will be to do so.
Good luck!