Depends I've just trained a newbie, never window cleaned before
started back in November 2016 he only does two days for me and two days for a friend from December
I started him with fan jets right from beginning and now his absolutely fine, I've been very harsh with training process, my wife says I'm bit strict
His just now working on finding properties and improving his English
I guess if you didn't have some watching over your method it would be less forgiving with fan jets but it's not unachievable