An often asked question on my Q&A's -
"How do you cycle assistance work"?
Let's start with the why...
First, if you use the same movements over and over, you'll adapt to the stimulus and stop making gains, bro.
Second, even if you're a freak and can get stronger almost weekly with the same assistance work, you'll start to develop imbalances or compensations to those movements, likely leading to injury.
Some general rules of thumb:
If you have been training less than 3 years, you can run the same assistance work for 6-8 weeks using linear progression or progressive overload. So each week add 5lbs to the bar, or add a set or a rep. To do this, obviously, don't max out a movement week 1.
This is an opportunity to master the movements. Changing them too rapidly will be tough for a beginner to practice these movements and eventually master them.
After 6-8 weeks, change your assistance work. Remember to pick movements that you struggle with or address hypertrophy needs. If you're unsure of what to do, focus on heavy rows, triceps, posterior chain, and upper back work.
If you've been training 4-9 years (a bit arbitrary here), then you may need to cycle assistance work every 4-6 weeks. As your training age increases, you typically adapt to movements more quickly, plus you don't get the 'newbie gains.' So it can be difficult to use linear progression or progressive overload week to week to make sure you're still progressing. It can be done, you just have to be smarter in how you approach your training at this stage.
You've likely already mastered some movements, so now is a time to explore new ways at accomplish similar goals. For example, I got away from push ups for a long time. I threw them back into my training in place of dumbbell work and was amazed at how much they kicked my ass.
After 4-6 weeks, reassess weaknesses or in a lot of cases be smart training around injuries. Don't let things fall by the wayside- especially if you're prone to avoiding things that you don't like. Embrace those movements and find ways to make a game out of them- one of the best ways is challenge training partners with assistance work.
If you've been training for 10+ years, you know the deal. All the newbie gains are gone. You have to bust your ass to add weight to any movement. You should know, at this point, what assistance movements build your main lifts, and what movements just plain don't work for you. You'll need to rotate assistance work every 1-3 weeks depending on what it is. Front squats to build your competition squat- you can probably do that for a week or two before needing to swap it, assuming you're going heavy with it. Upper back work to finish a training session? You can probably push that for 2-4 weeks depending on how you program it.
*In most cases, the more 'athletic' an individual, the faster you need to rotate assistance work. Mainly because that individual is able to adapt or even compensate to certain movements more quickly.
Thoughts...
Once you've adapted, meaning once something becomes 'easy,' then it's time to change the movement.
Once something becomes 'too hard,' or you can't continue to improve on it, then it's time to change. But don't forget to revisit it in 1-2 cycles.
There is a case to be made for once something is no longer exciting, you can switch it out. Now don't make this a habit. But if you find yourself bored, don't feel bad about doing something new and different, as long as it makes sense within the context of your program.