It is good to get into a routine of working out. Most people do well, if they workout the same days of the week at the same time every week. It just becomes a habit. What isn't good is doing the same workout day in and day out.
At first, you will notice changes in your strength, endurance, and weight loss. After some time though, all of that usually seems to slow down. Once people don't notice results, they will likely stop the exercise. The changes stop or slow down, because the body is easily adaptable. It doesn't take long before what used to be hard, is now easy. You won't have to exert the same amount of energy, because your body knows what to expect and how to react. Thus, leading to burning fewer calories. Not only that, but doing the same thing everyday, leads to boredom.
So, what can you do to keep your body guessing, and burning optimum calories? Change your routine! You can do this in many ways. Find a new class, or exercise that you really enjoy, and start incorporating that into your routine. If you always do the treadmill for 30 minutes, at 3.5 mph, incorporate HIIT (high intensity interval training). These are short bursts of a high intensity exercise. For example, 30 - 60 seconds of running, or hill climbs. You then return to your previous level, before doing the same interval again. Your heart rate will be going up and down, which will be burning more calories. I like to do a 2/1 ratio. If I run for 1 minute, I walk for two. You can also do this with weight training. After a set of 3, I will do a 1 min cardio interval, such as running, burpees, boxing, or jump rope. Don't just change up your intensity, change up the exercises, reps, sets, and weight as well. Just don't change all of those things at once. Focus on one change per workout.....and ladies, with weights, don't be afraid of them! Don't be afraid of lifting heavy either. You won't end up looking like The Hulk. Most of my workouts are weight training. I would never have the shape I desired, if I did cardio only.
So, how often should you change up your routine? I do a different routine everyday, but I would at least change it up every two weeks. So, keep boredom at bay, and keep that body giving you the results you desire and are working so hard for!