That is a very good question. That depends on many factors and to put it simply those factors are:

• the dog himself/herself
• the owner
• the motivation of both
• the personal goals and level of proficiency that the owners aim for
• the existing behavioral issues that need to be worked on, their severity and intensity (in other words: there's a difference between solving an issue such as leash pulling compared to some much more serious issues such as aggression related behavior problems, etc.)

Here's a short analogy. Let's say somebody asks how long does it take to learn how to play an instrument... That's very much like training a dog: it does take practice, it depends on the motivation of a learner, on the parent as well - if it is a little kid that needs to be supervised about his musical practice, the goals of a learner - does a learner want to simply learn how to play one or few songs (or in dog situation - does the owner wants for a dog only learn a few behaviors?) or maybe the learner has such love for music that wants to be a professional musician or at least become very fluent at playing music...?

You, as the dog owner, are the facilitator and motivator for your dog and you can encourage your dog to do more and more, but as far as the answer to the question goes - that's up to various factors and it's hard to pinpoint the exact time frame because "training" means various things to different people.

Here's a visual example of what CAN be done (but again, every dog is different) in only 2 weeks of Board and Train program with a puppy that did not have any previous formal training:

Dogs learn their whole life and in this sense, a dog always learns SOMETHING: either something good or something bad (if there's lack of proper management and gudance on the owner's part). In this sense, training is a lifelong process because even when a dog acquires new skills (that's only 1 phase of training process), maintaining and practicing those skills are other very important phases of training.