Frequency is one of the most important variables for growth, but I handle it completely differently now than I used to.
I don't apply high frequency to all areas concurrently, this is not really that enjoyable when you're going well beyond the intermediate stage unless you are a masochist or some kind of warped misanthropic sadist.
These are my preferences for frequency now based on the programs I've been running. At this stage, these are the principles I rely on, regardless of the program:
[1] Use microcycles to specialize training towards weak areas that need attention. An example of this can be doing legs 3x a week whereas everything else gets trained 1x a week. Do this for about 3 weeks and then run the next block where you train another area with this level of specialization.
[2] Auto-regulate training. If you train for bodybuilding, you have no excuse not to. Use RPE scale to determine working weights.
And this is where I deviate completely from what my thoughts are as an intermediate...
[3] During a high frequency phase, you should split up the kind of work you're doing so each important aspect of the area you're training has it's own session'.
For instance, during a lower body specialization phase, Session 1 can be Quad dominant, Session 2 can be Hamstring / posterior chain dominant, and Session 3 can involve direct glute work.
The reason for this is actually pretty simple: the more you split something up, the more intensity you can apply in training. Within reason though, and not at the expense of frequency.
I like to increase the intensity gradually as the weeks in the micro-cycle go by and once it's over you dissipate that body part(s) accumulated fatigue as the next specialization phase begins, but towards the end of the micro-cycle, I would also incorporate movements on the secondary and tertiary sessions that have some carry over (ie. doing a movement on hamstring day or glute day that might involve your quads as well, such as lunges or split squats)
This is to boost MPS, and you don't need much crossover between sessions at all, and I would probably only do that for one week, because the weekly volume you are doing should be more than enough for growth, as well as the fact you are splitting up a single aspect of your physique even more so you give it more attention.
[4] Exercise selection will be specific, no redundancy. If it doesn't have a purpose, it does not belong, you are better off doing more sets of a movement you like more if there is no discernible difference or connection between movements.
