It's completely different between individuals as to how and when one should change their workout template...but the guidelines instructing how to change a workout plan are the same for most people!
If you're new to exercise it is suggested that you keep the same workout plan for 1.5 to 2 months, but if you're advanced 2 weeks or even less is suggested as a guideline. It's said that if beginners change their workout too often it'll be harder to gain the motivation and it'll feel like less of a routine to perform your workout.
w. No Change: you can hit a plateau that's tough to break
w. Too Much Change: you can hinder your results and it can become tedious
...if you're completely changing over from one program to another (aerobic training to weight training for example) it's important to take a week off in order to give the muscles and nerves a chance to fully recover!
What To Change?
...sets/reps, exercises, intensity, amount of days, length of workout
- sets and reps...
...if you start to plateau then increase the sets and decrease the reps or vice vera
...the total volume is the same but for weight training it increases the total weight lifted
...can be changed every 3 to 5 weeks, or as soon as you see an adaptation/plateau
...variety is key, but it's important to outline your goals (short/long term), and to monitor them so you can see how you're progressing
...can be changed day to day (for example: one low intensity workout, one medium, and two high intensity per week)
...greater gains can be experienced instead of simply avoiding plateaus
...it's important to include the cardio portion of your workout in frequency changing