一億年ボタンを連打した俺は、気付いたら最強になっていた　～落第剣士の学院無双～•Ichioku-nen Button wo Renda shita Ore wa, Kizuitara Saikyou ni Natteita: Rakudai Kenshi no Gakuin Musou

I Kept Pressing the 100-Million-Year Button and Came Out on Top: The Unbeatable Reject Swordsman

Allen is training to be a swordsman, but he's just no good at it. After losing a duel with high stakes, he's about to drop out when a mysterious man offers him a chance to get 100 million years' worth of training with the press of a button. But what happens when he presses it over and over?! (Source: Yen Press)