Setting Up - Basic Requirements
Your startup costs for getting into a really productive digital photo retouching work environment are not going to be considerable. You'll need to assemble a relatively inexpensive collection of hardware and software items, nothing much above the normal Windows PC microcomputer installation for home or small office.
Learning the software - I've also recommended only software which is easy to learn and to use, and which have very clear and intuitive user interfaces (command and working screens). You can master the real power features of most of these programs within an hour or a very few hours. Then it's simply a matter of practicing on example photos for a short time, to get the hang of the individual operations.
"Combining" the software - I strongly advise "mixing & matching" the best processing features of a number of the programs. That's the basic DPM idea. You'll quickly learn which of the software packages is best and easiest and fastest at doing the particular things you need to do to a particular image. You'll soon have favorite tools to use for removing spots from images, adding colorization to black & white images, removing (or adding) objects from an image, lightening the image, improving contrast, etc. You'll also know when to use the slightly more complex software to handle the more serious image problems, and do them much better than you can do with the really "quick & easy" tools.
Time requirement per image - Of course, this depends on the "amount of fixing" you have to do to a particular photo image. I would guess that my photos generally run from the 2 or 3 minutes range up to about the 20 minute range. I'd say my overall average retouching time is about 10 minutes. The longest I've ever spent on a single image is about an hour and 15 minutes.