Dan the time involved can be extensive but it can be fun. A retriever will retrieve with his "natural ability" and are always willing to please. It's getting the dog to do what you want when you want that can be difficult and that's usually not the dog's fault but the handler due to inconsistency. I would suggest you read all you can and if you've never had a hunting dog a good book to start with is "Raising a puppy you can live with". It give you some basic understandings of dog psychology. Another thing I would suggest is to join one of the retriever clubs. It will get you around other novices as well as experts and helps keep one motivated.

I have trained all my own dogs(two retrievers and 5 pointing dogs) over the yrs.
I start with obedience in the backyard then take them to an obedience class to further reinforce what they know but around other dogs with more distractions. Once they understand what I want I just continually reinfoce those commands in the field. Their natural ability will get the retrieveing and pointing done.
With retrievers you'll read a lot about handling but with most hunters and hunting situations it isn't necessary. That's for the guys who compete generally and want to take dog training to its highest level.

How much time is involved---depends on how much you want to devote. If you don't have the time or are not willing to make the commitment reread what antlerman posted above.