A quick illustration with my squirrel model.
The model had been made for animation purpose, the mesh is pretty lowpoly, around 6k and it uses about 220k hair prims.
For an HD rendering it takes about 3 minutes, the original hires image is 3k and it took about 15 mins.
Modelled with Maya, rendered with MentalRay.Shave and a Haircut for fur.
The grass is fur matched with the background photo.
Tools: Maya, MentalRay, Shave and a Haircut