I get this one all the time from people who are just starting out.  They want to know..
Do you ever wonder why so many people join a gym in January and then it’s business as usual the end of February or mid–March?  There’s a couple of reasons.
First, working out is something you do as a habit.  Not just something you do for four weeks.  Many of the people who join up, have no idea what’s going on.  They just sign up.  Hand over a year’s worth of dues and get on a machine.  The lucky ones get a 30 minute overview of the gym with a personal trainer.  And maybe the trainer is good.  And maybe not.  They have somewhat of a goal.  Lose weight.  Get bigger.  And they don’t have any idea of what their body can do.  After about 4–8 weeks of this, it’s old.  Not exciting anymore.  They don’t really like exercising anyway.  Now going to the gym is a chore that ranks with paying bills.  The excuses start (have to work late, pick up the kids, my <insert body part> hurts, I’m tired) and it’s over.
Second, if you don’t see results fast and I mean FAST, it’s over.  If you don’t lose 60 lbs. in 4 weeks, then working out has failed you.  If you didn’t gain 40 lbs. of muscle in a month, then you are a hardgainer and will be doomed to lead the life of a skinny guy forever.  There’s a whole group of people that want to obtain a certain body image.  But 16 years is way too long.  They want to be slim and trim or big and buff in 2 months for some occasion.  Or maybe they saw all the ads over the holidays, looked at themselves and said, yeah why not.  I’ll just join a gym.  Fitness is easy.  Nothing to it.  I’ll do a little here and there.  I’ll do cardio.  I’ll quit eating.
They quit too.
How long after working out do you see results?
On average, if you change your diet, follow a moderate to intense routine and introduce some interval training cardio, you will see results in as little as 4–6 weeks.  Nothing earth shattering mind you but changes.  Slow progress.  You might even start to feel internal body changes within a week.
The key to getting the results you want as fast as possible is to monitor where you are now, where you want to be then and what you are going to do daily to get there.
Those that think results only come at the gym really never see results.  And they quit.
Those that understand fitness is 24x7x365, will see results in 4–6 weeks.  When you wake up and eat breakfast or don’t eat, that is fitness.  When you consistently don’t get the rest you need, that is affecting your goals.  When you have no way to measure progress, it’s very difficult to tell if you are going anywhere at all.
Just because you cannot “see” something does not mean it’s not there.  Every time you pick up a weight or get on the cardio machine, jog or run, you are making changes in your body.  You might not see it but I bet you can feel it.
Bottom line, there’s no magical number of when you might see some progress of your efforts but on average, in four to six weeks you can begin to see your efforts pay off if you understand that fitness is a lifestyle and not just another diet or thing you do when you have the time.
Here’s some other suggestions you might be interested in reading.
How Long After Working Out Do You See Results?