Depends on when. I've made ressies 10 days out (in late April) and even on 24 hours notice (late May/early June) to extend a stay. If you tried over Christmas, early December, or during the Food & Wine Fest it might be dicey.
One thing to keep in mind is that usual "slow" times for WDW usually don't coincide with the "slow" times for DVC. One of the slowest times for WDW is the few weeks after Thanksgiving leading up to Christmas. That is probably one of the busiest times for DVC, with basically no availability to be found. Now, on the other hand, summers are VERY busy for WDW, but DVC can be slow at that time. I also agree, if you plan on traveling in the fall (Food and Wine festival) or the first few weeks of Dec, you'll probably find limited availability during that time so close.