The first time I was on Prednisone, I started at 20mgs/day and it took me almost 4 years to taper down and finally get off it completely. I found that once I tapered down to the 10mg range, even .5 or 1 milligram made all the difference in the world as to how I felt that day. I usually tapered down by .5mg-1mg at a time.
Even though I had to go back on Prednisone this past May (at 20mgs/day) it has only taken me 3 months to taper down to 2.5mgs/day - A LOT quicker than last time. I'm not sure why, though.....
Good luck!