DVC resorts tend to run at (or very close to) capacity all the time. The difficulty in booking varies and it really does depend on WHEN you call to make the reservation. In other words, it is hard to answer your question without knowing how far in advance you plan to reserve. Weekdays do tend to book up more quickly than weekends, but the weekends also book up.
Peak times for the DVC resorts do not necessarily coincide with peak atendance in the parks. For example, Thanksgiving week through the end of the year are very popular with DVC members. It's rare to be able to get last minute accomodations during this time - you will read reports of people doing so, but it is NOT the norm for most. In fact, I believe availability for the rest of this year is very limited.
The EPCOT resorts are harder to book during the Food & Wine Festival. Holiday weeks are popular at all the resorts. The summer weeks are very popular at Hilton Head.