I try to encourage my students to constantly adjust their focal length, based upon the speed they are traveling. The faster you go, the further ahead you must look.
It also will depend greatly on the snow conditions and terrain you are in, as stated by previous posts. In more difficult conditions, I tend to shorten my field of vision somewhat, but never real short.
With the current crop of skis we use allowing some unique directions of travel, please don't forget to look around yourself. And I mean ALL AROUND! Including uphill, as well as out to the sides. For your safety and everyone else on the hill, be aware of the other skiers movements around you.
One of the critical issues as to how far ahead you are comfortable looking is based upon your own skills. The more confident you are, the further ahead you will look.
Oh ya- as already stated- don't forget to look at the scenery. (and the mtns too!)