I've mostly been an enthusaist, because I could never get a serious routine, and keep it going, so I never really achived much in terms of personal muscle growth, but I had times were I did train pretty consistent. But even though, the science of bodybuilding was and is a big interest to me.
Mentally, I think I want to cling to the "less is more" idea, like what Mentzer and Jones taught. I like that, and it makes sense to me. But, when I look back at the various training ideas I tried, and I did try any new or radical thing I read about, usually, one time really stands out, because I was making large strength gains, and was seeing noticable changes in my physique, and it's the only time strangers would comment on my size. And while, common recommendation would say to either do a full body routine every other day, or on M-W-F, or split it in 2, and do a M-T and T-F, or split it in 3, and do it 6 days, then some proponents like Menzter would say do basically one rep to failure, but do it like once every 4 days, or once a week, emphasizing the need for adaption. But, what I did was totally different, and here is what I did:
A full body routine, of one main exercise per body part, and 1 rep, 1 set.
An example, say for curls, I'd load the bar, do one rep, light? add more weight, and keep doing that till I had one hard rep done. Then I'd write that weight number down. I did the whole routine like that, and this was the start. Now, I'd go back to the gym the VERY NEXT DAY, and add some weight over what I had on the bar, maybe 5lbs or less, and just crank out one rep, and it worked, in that each movement gained in strength, rather rapidly, and my visual appearance was changing. Now, looking back, I would change some things. I would do the same, but increase the reps, to maybe 10, or higher, maybe 20. Or, if I wanted to keep the reps low, I'd do it for 4 days straight, say M-Th, and then take off till next M, and repeat. But, I think with higher reps, still, to failure, I think once could do M-F with weekends off, or maybe M-S with sunday off, or try everyday.
I'm also thinking frequency and rep range might also depend on one's body type. I've always been on the fat side, yet there was a lot of meso under it. I'm thinking an ectomorph might do better with a Heavy Duty type routine, few reps and sets, and a good amount of rest time in between sets. A more mesomorph, probably would do well with medium reps, and maybe 2 times a week for body part, but an endomorph might do better with higher reps, and higher frequency of body part training, like daily, as I explained above. As far as sets go, I still hold to the Heavy Duty idea, that if the muscle is worked to failure, one is enough.