There's interesting research showing that the progress you make on an exercise that is relatively new to you is mostly because of skill learning and neuromuscular adaptations, rather than an increase in muscle size [1].
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Once you have learned the skill of a movement, you will be able to complete it with proper coordination of relevant muscle groups and start maximizing muscle growth [2]. This is why constantly changing up the exercises you do for "muscle confusion" is not necessarily a good thing. You'll have a hard time mastering the movements you do for muscle growth.
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So try to keep the exercises you do in your training somewhat consistent. As a general rule of thumb, I would aim to keep the main compound exercises in your routine for at least ~8 weeks. To prevent your training from become repetitive/boring, you can change up the simple isolation movements in your routine more frequently (every ~4 weeks).
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That said, there are instances in which it is fine to change up your movements more often. If a certain exercise feels weird or uncomfortable, it's perfectly reasonable to opt for a different variation after a few weeks.
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References:
1.
https://www.ncbi.nlm.nih.gov/pubmed/9459538
2.
http://jap.physiology.org/content/102/1/368.long