well, Drago is constantly evolving because he is the main character, but actually Drago evolved more in the first season. It seems like that because of the rate of evolution and evolution of everybody else. Season 1 Dragonoid episode 1-22, Delta Dragonoid episode 22-37, Ultimate Dragonoid episode 37-51, Infinity Dragonoid episode 51-52 Neo Dragonoid episode 1-27, Cross Dragonoid episode 27-42, Helix Dragonoid episode 42-52 If you look at the path now, he is evolving faster Helix Dragonoid episode 1-12, Lumino Dragonoid episode 12-29, Blitz Dragonoid episode 29-39. Drago evolved 3 times in the first series, but only 2 times in New Vestroia and Gundalian Invaders
Then there is the fact that Dharak only evolved once so far, nobody else even evolved. If Drago evolve 2 more times, then you can say he evolved every 10 episodes. and there is a chance he will evolve again.