Like Bruce wrote, it can be 5-30 minutes or many hours depending on the how you want to present your work. Allot of post-processing can be avoided if you get the image as right as possible in the camera. That means getting the background, lighting, and camera settings as close to how you want the picture to look as possible. Getting as close to how you want the picture "in-camera" to look frees you to just touch-up in post or spend the time being creative.
My typical convention photography routine is:
1) Shoot in RAW format
2) Import to Nikon Capture NX2 for minor photo corrections and jpeg output
3) Import to Photoshop (if needed) for minor blemish corrections
Usually 5-8 minutes a photo.