This is a question clients put to me all the time; unfortunately, it's hard to answer. That's because, even though I know they're looking for a quick answer like "4 months" or "8 weeks," the truth is it depends on a lot of different things.
First, there's your background. Are you a former athlete used to hitting the weights, or a reformed couch potato who is new to working out? Your past and existing fitness levels can dramatically affect the time you need to get up to speed.
Plus, everyone has a different definition for what "in shape" means. To some, it's being able to play in a tennis tournament or run two miles without stopping; for others, a marathon, or just looking good on the beach is the goal. Without having an idea of what you want to achieve, it's tough to know what kind of progress you're making.
With that fine print laid out, however, I'll offer an answer that's probably closer to what you're looking for: it takes most people, on average, about 10-12 weeks to start seeing noticeable gains in their fitness program. They might start to feel more energetic right away, or detect changes like weight loss earlier, but 2-3 months seems to be the point where most people can tell that their physical appearance is changing.
But, as I mentioned before, you may or may not think of that as being "in shape." If your goal is to compete in a running, swimming, or biking race, you might want to plan for closer to six months, or a bit longer if you're shooting for something even more ambitious.
There are probably two other things I should point out here. The first is that you can't rush results. Like a "crash diet," a workout routine that doesn't allow enough rest or recovery isn't going to do you much good. In fact, it could lead you to a very serious injury or illness, so resist the urge to go above and beyond what your doctor or trainer recommend.
And secondly, remember that your time working out is just part of the equation. Working hard is great, but it won't do a lot for you if you have terrible form, or if you finish your exercise by eating a box full of sweets. Rest and nutrition are very big parts of the process, and how well you incorporate them along with your strength and cardio training can easily mean the difference between making progress and hitting early plateaus.
Even though there might not be one set answer to the question of how long it takes to get in shape, there is one thing I can promise you: if you make a commitment to working on your fitness, hire a quality trainer to help you get started, and take advantage of your time to rest and eat in a healthy way, then it won't take more than a couple of weeks for you to start feeling better than you have in years.
