For example, if you have a 1 or 2 trainer and you train just C you will get a pop in a week with good minutes, but if you train multi positions it will take 2-3 weeks.
I'm not sure I follow your message. I've never done single position training, but if it takes only 1 week to train a skill with a level 2 trainer, I might start.
Also, I'm pretty sure it is not possible to pop 2 levels in a given skill, with only one week of training.
Run of the Mill Canadian Manager