There are some people who thinks Naruto learned rasengan in a week. It seems that they forgot the other 2 steps (the water ballon and the rubber ballon) that Jiraya taught Naruto. Seriously how long did it took him to learn rasengan? Some said 1 month...
On a side note how long did it took for Boruto to complete these 2 steps? Too bad he didn't have the "never give up nindo" to complete his training, he gave up to easily and he had to cheat. At the end of movie, didn't he do on his own a normal rasengan? I guess it's safe to say that he mastered the rasengan