This question has a lot of "it depends" answers. On average I will only spend about a 1 minute on each image. Most images are less than this (20-30 seconds) but I'll have the occasional image that I'll spend 5-10 minutes on if I really like it and want to do more involved retouching/editing/enhancing. I probably spend more time culling my images than I do editing them. (I'm admittedly slow, it usually takes me 60-75 minutes to cull a 400-image shoot down to the final 40 or 50 I present to a client)
I do 90-95% of all my editing and retouching in Lightroom. I only use Photoshop for the really detailed stuff (major retouching/cloning, composite images, texture overlays, etc.) Adobe has done a really good job of making a lot of the features I use in PS available in LR. I rely on presets and use their tools for selective dodging and burning quite often. LR also makes it easy to apply settings to similar images very easy (Previous button in the Develop module and Sync Settings in the Library module). These can be a big time saver.
When I first started several years ago, I was definitely spending more time than I am now on each image. At least 5-10 minutes. Usually it was because I was trying to rescue an image that I hadn't captured in camera correctly. Sure I wrote actions in PS and presets in LR to help, but in reality the best option is to work at improving your ability to make sure you get it right in camera first. This pays much bigger dividends on the back end than anything. Even someone just starting out can do a few basic things to help in this area; you don't have to be an advanced user or a professional.
Suggestions of things that have helped me
- Understand the basics of a good exposure (Aperture, Shutter Speed, ISO). Then take a little extra time to position your subjects to have good light on them (I still use the technique to hold out my hand to find the light). As an example, find good open shade when the lighting is harsh. Good lighting trumps a good background any day.
- Learn how to use M mode on your camera. I used to use Av but I would end up with all kinds of different exposures, even for the same scene. This is a pain because it takes longer in post processing since there's not consistency between even similar images.
- Use a reflector. Seems simple but can altogether eliminate the need for having to try and create digital fill.
- Get your white balance right in camera - Either use a digital calibration target (I use ones from PhotoVision) or manually set your white balance. This takes some time to learn, but establishes consistency between images. The AWB on Canon's cameras aren't known for being awesome.
- Value quality over quantity. Shooting more doesn't yield a higher % of keepers. Take the time to chimp your shots. Develop a critical eye for details -- it's much easier to simply have your subjects fix hair fly aways, toss their gum, and straighten wrinkled shirts and take another picture than it is to spend hours trying Photoshop it out later.
This is certainly not an exhaustive list, but they are things that helped me and made a huge difference in helping to reduce time spent on the back end doing PP. My time is valuable and I can't be as effective or profitable if it takes me forever to PP all my images, especially with weddings when we come home with 2,500-3,000 images from those events.
Hope this helps. Like anything, with more practice comes speed and efficiency.