How often should I change my exercises?

Direct answer

You don't need to regularly change exercises just for the sake of variety. Change them when there's a reason to.

Explanation

Keeping exercises consistent gives you time to improve your technique, become more confident with the movement and actually measure whether you're progressing.

If you're constantly replacing exercises, it becomes much harder to tell whether you're getting stronger or simply getting better at a completely new movement every few weeks.

That doesn't mean you should perform the same exercises forever.

There are plenty of good reasons to make a change. Your goals might have changed, you may be ready to progress to a more challenging variation, an exercise might no longer suit you, or you may want to place more emphasis on a particular muscle or movement.

A prolonged lack of progress can also be a reason to consider changing something, but I'd look at the bigger picture first. Are you training the exercise consistently? Is the effort appropriate? Are you recovering well? Is your nutrition supporting your goal? Has your technique improved even if the weight hasn't?

Sometimes the programme needs changing. Sometimes it doesn't.