we went last year and had to be diverted away from Sofia due to the snow.. then it took 4 hours for the coach to pick us up then about another 4 or so to the hotel! got there bout 4 in the morning in the end!
once you get there however you will have a fantastic time but try to forgot about the transfer time...
We have done the journey many times,
We were in bansko last week, On the 23rd dec from sofia to bansko it took us nearly 4 hours but there was snow and ice on the roads. However driving back to sofia on the 2nd jan it took us 2hr 10min ( record time ).
The prevoius times it has being somewhere between that.
Hope this helps, skidude