from what i believe it will take probably an entire season to get a pop
i have a 25 year old mvp and i have been training him all season with (besides stamina) a total of 1 pop for the season - that pop took 5 weeks and was in od and now i am trying for a passing pop - i used a level 6 trainer all season- although i do believe what everyone says that the difference between level 5 and 6 is somewhere between 2-5%
i would only train a 28 year old if i had to against my will - for example he is the teams best pf and you are doing rebounding and you will lose games if he doesn't play