Depends on how serious you are about your DH racing. Training for DH is very complex, much more complex then training for XC or road racing.
The key to success in DH is a high a skill level. Nobody is born with a high skill level, everybody acquires it through practice. The more quality practice you have the better you will be.
This is where fitness comes in. Most people have a fitness level that allows them to do at most 1hr of quality practice in day. Their training day may be longer but they are lucky if there is an hour of quality practice. People with a high level of fitness are capable of more quality practice, more quality practice means more skills.
Of course their are many ways to develop fitness. What you do in the winter to improve your fitness is quite a bit different then what you do in the summer.
The winter is the time to work on your aerobic system. The aerobic system play an important role in any event longer then a minute and a half, it also allows quicker recovery from maximal efforts.
There are tons of ways to develop the aerobic system, the best way to do it is debated constantly among endurance coaches. The traditional way is start with long steady duration and then proceed to shorter and harder workouts.
So…..
Depending on your back ground if you are serious about racing you should be doing 4 to 6 aerobic workouts (road rides, XC rides, XC skiing, or running) that are 1 to 3hr is duration per week. Do this more two months or so and then proceed change to a bit more intensity.
Some time should be spent in the gym to develop strength to help you handle crashing. A lot of time in the gym is not necessary however, there is very little correlation between gym strength and riding speed.