My cut off is 40 mins. If the training im doing somehow ends up stuffed up for whatever reason, and my player gets more than 40 minutes for taht position, i keep my training as is. Otherwise I switch to something I need. Need being the key word.
Logic being that id rather 40 mins of One position training, than 48 minutes of 2 position training in rebounding, or Jump Shot....... OK so maybe you dont get 0.8 of a pop towards Inside Shot (or whatever it is), you get 0.7 or 0.6 (if he played 40 mins) Id personally rather take training in the skill that you DO want trained, rather than just get some random pop in a skill that doesnt need it.
Depends on circuomstances and skill levels of other skills of course, but thats just my tactic. Especially with older players, whos stats are quite higher already and your looking for that 'specific' skill- pop.