I’ve heard that confusing the muscle is a great way to cause growth but I don’t really know how often to switch up exercises or reps.
I’d say the majority of moderately experienced gym goers can use similar exercises and rep schemes for about 3-4 weeks tops before needing to switch things up. Younger bodybuilders can stick with similar protocols for about 8-12 weeks before switching things up.
If you are a highly experienced gym goer, the more often you can place a new stimulus on your body, the more often you can continue to adapt to training. Highly experienced bodybuilders should add in training variation every 2-weeks at the most but many may even require weekly changes.
Things you can vary include: set and rep schemes, rest periods, exercise selection, equipment used, order of exercises, and training goals.
Fleck, S.J. (1999). Periodized Strength Training: A Critical Review. The Journal of Strength & Conditioning Research. 13(1): 82-89
Rhea, M. R., Ball, S. D., Phillips, W. T., & Burkett, L. N. (2002). A comparison of linear and daily undulating periodized programs with equated volume and intensity for strength. The Journal of Strength & Conditioning Research, 16(2), 250-255.