When to change things up.

When it comes to exercise selection, it’s sometimes believed you should train for “muscle confusion” and constantly change up your exercises. But if muscle growth and/or strength gain is the goal, consistency in your compound lifts is beneficial.
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There’s interesting research showing that the progress you make on an exercise that is new to you is mostly because of skill learning and neuromuscular adaptations [1]. When you have mastered the skill of a movement, you will be able to complete it with proper coordination of the relevant muscle groups and start maximizing muscle growth [2].
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This is why changing up the exercises you do very frequently is less effective. You will stay in a “learning phase” in which muscle growth is not optimal. A rule of thumb I like to maintain is keeping the main compound exercises in your routine constant for at least 8-10 weeks. An exception is when a compound lift starts producing discomfort or you really dislike a certain compound exercise.
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Now, if like to have more variation in training, you can vary your isolation exercises more frequently. Isolation exercises like side raises, biceps curls, etc. are more simple and require less “learning.” Switching up these exercises every 4 weeks or so is a good rule of thumb to maintain here.
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References:
1. https://www.ncbi.nlm.nih.gov/pubmed/9459538
2. http://jap.physiology.org/content/102/1/368.long