I don't usually set mileage goals for cycling but I did so at the start of 2015. That's because I came very close to topping 5,000 miles commuting and 10,000 total miles during each of the past two years, and I thought that setting those numbers as goals would put me over the top. It didn't work out that way. Weather was a big obstacle for commuting this year, starting with two weeks of icy roads in February and continuing with the most rainy days we've had on record going back to the 1800s. I should end up the year about 150-200 miles short of my commuting goal, or about 4750-4800 miles, which is not bad considering the weather. When I subtract out all of the days I was off work due to vacation, holidays, comp time and sickness, I averaged about 110 miles or 3.5 commute rides per work week.
I do not plan to set any mileage goals for 2016. The mileage will take care of itself. I don't want to pressure myself into riding on days when I need a rest or the weather is lousy just to top some arbitrary number. In general, I think goals are a good motivator for performance. However, weather and other work demands play such a huge role in my commuting that setting mileage goals is kind of pointless.