If it helps any, I typically change or alter an accessory movement when it stops improving.
For example, someone may get Incline DB press. 3 x 8. For simplicity, lets say for 3 weeks they go up in weight, then on the 4th, there's no progress. I'll then switch it. It may be something like, one arm DB bench press. The reason is, if you've never done it, you'll be bad at it, if you're bad at it, you'll improve quickly, and so on. Then when you get back to doing things you've already done, in theory, you'll be better in some way (very short explanation, really).
In other words, I don't think of it in terms of "beginners should stick to the same for 6 cycles, intermediates for 2-3 cycles" or make any effort to comply to any kind of standard. I just pick things we can improve on and when we stop improving, we pick something else. I like to make people uncomfortable, and force them to get comfortable (adapt). I like people to be adaptING but never "adapt".
The main reason is, back to Incline DB press for 3 x 8. If you're doing it week on week with the same weight for the same reps with the same rest at the same tempo... then what are you getting from it?
In theory this means that myself and everyone I train should be doing every exercise variation with record breaking loads but, obviously, it doesn't work like that. The more advanced you become the more you need to mess with things but, the above seems to work really well with the intermediate level lifter, and amazingly for the "beginner intermediate" level.
In theory it this could also mean that you pick things just because they're difficult, which kind of paves the way for one-arm-shake-weight-curls-whilst-standing-on-one-leg-on-a-bosu-ball-balancing-a-Vipr-on-your-head, just because you're bad at it. However, there should be more reasoning than that i.e. my left arm always gives out first and i'm quite unstable, so i'll go for 1-arm DB press for 4 weeks just to work on that, and i'll put a little more into the left side.