To put some of this in perspective - I've been more sore training 2/3 days per week and eating to lose weight than I have been lifting on a 40-day Dan John plan 4/5 days per week and eating to gain weight. So in my experience, soreness is much more about the quality of recovery (nutrition and sleep) than about the frequency of training.
To Forever Young:
If you need a full week to recover and gain reps then either a) you are already pretty strong and that's just what it takes, in which case 15 minutes workout isn't nearly enough, b) you are not eating enough to recover properly from workouts or c) you are leaving a lot of gains on the table and could make double or more the gains you are currently making. Also, I'm not sure why whatever someone is currently doing has to be the same thing they'd be willing to do for their entire lives - why can't someone spend a year or two getting in great shape and then switch to a less taxing maintenance program?