You might know it’s time to make a change if what you’re doing is no longer providing results. Be realistic here, based on your goal, are you progressing?
If not it may be time for a change.
Also, depending on your goal, whether it’s muscle building, strength or fat loss, your program should reflect the changes to your goal.
If you get bored with your workout routine, or become unmotivated it’s probably time to make a change to help bring back interest/ desire and motivation.
With that being said, I never suggest changing routines after two or three weeks, you definitely didn’t give yourself enough time. Again, be realistic and focus on the big picture.
Lastly, if something out of your control changes. For example, if your schedule changes or you occur an injury, you have to adapt your workouts. You may have to adjust your workouts from say five days to four days because you don’t have the time you once had.