Board Thread:Theories and Speculation/@comment-31266380-20170213121035/@comment-734582-20170214074538

Lorenzo.r.1st wrote: not enough time.... hum... interesting. three years to make a jutsu outta nowhere (rasengan), which he didn't even if it was accomplishable, instead of mastering his sm, which is a jutsu that doesn't take almost any of ur chakra, and makes u at least 10 times more powerful. so do u think that a prodigy/genius would've preferred making the rasengan or learning sm? sm by itself could've made any of his other jutsu just as powerful as a basic rasengan. this is plot imo. Who says that he wasn't learning both? But we've seen the activities of three Hokage now, and all three were continuously busy with paperwork. Fans assume Minato was training every day continuously, whereas in reality he was busy with a war, after which he became hokage and father. I doubt he was able to put much time into developing an elemental rasengan or a Sage Mode.

Secondly, we have no timeline on him achieving sage mode. For all we know, he achieved a Perfect Sage Mode 3 days before Obito's attack and did spend all those years training. Hell, he may actually have put the Rasengan on hold to develop Sage Mode.