Length of daily meditation (Soto zen)

Most Soto Zen sources that I've seen advise daily Zazen, preferably when first rising and before going to bed. However, few make reference to a length of time. The only one I remember says half an hour. At this point I feel like 15 minutes would be a serious challenge (I am a beginner). Is this a problem on my end, or could I perhaps start with 15 and work up? If so, at what rate should I increase?