Time vs. Distance

I am sure this has been asked many many times, but I looked through the first 3 pages and didn't seea thread on the topic.

I am just starting the sprint winter program. For the swim there is both a time (12 minutes) and a distance (400). As a general rule, which should I try to follow? IE. If I am really slow should I besure to get the distance in, or if I am fast should I continue to swim until I have met the minutes.