Not very definitive, I'm afraid, but here's some anecdotal evidence from training jump shot for my entire team - they're all relatively young, and at the start of this season my shooting was so poor I decided everyone needed training.
After four weeks of training JS for the whole team, I've had 11 pops. 9 in JS and 2 in JR. One player (playing at SF but with skills for a SG) has popped twice in JS in the first and fourth week, but no-one else. Only a couple of players have popped in both skills. No other pops. Eight of the pops happened to players who'd accumulated more than 48 minutes in that week, three to players with less (28, 37, 41) - which to me suggests that the penalty for getting less than 48 minutes isn't that great.
With a squad of 16 players I've had some not see any pops at all, but my team's shooting percentage has improved significantly, especially in taking 3FG.
The only player who has consistently received more than 48 minutes yet not seen any pops is 24 years old. It's a shame cos he's my best PF and gets the most minutes every week!
Clearly it's difficult to compare like with like, given that each week I play different opponents, but my 3FG percentage the week before I started this training was 13%. SInce then it has been 31%, then 25% then 35% then 23%. Overall my shooting has improved from around 30% to nearer 40%. This week I've switched to 1v1 training, and despite playing the top team in England yesterday, we shot 25% from outside the arc, and 37% overall. We lost, but I can't complain.
My inside shooting is now the issue which is letting down my team, so the experiment has to end while I switch to other training.
My conclusions are that it is worth training the entire team, if your entire team is young (or nearly). There's some kind of variable in the training speed, since one player popped JS at a rate of 3 weeks, but no-one else did, and some players didn't pop in 4. However, most players did pop in at least one skill during 4 weeks, suggesting that pop times should be on average less than 8 weeks, certainly for JS. If you want to improve individual players for selling this clearly isn't profitable, but if you need to improve your whole team in order to compete in your league I think it is worth doing. I know that's controversial, though.