If memory serves, Ultear got him out of the prison pretty soon after she returned, with Meredy, from Tenrou Island, which means he has had time to train for most of the skipped time. He wasn't locked up for any significant amount of time in that sense. Although probably enough to know he doesn't want to go back to be tortured by petty frogmen who love to punish him for their own shortcomings.
I suppose Millianna might have some chances to evoke a weakness in Jellal, but I have a feeling he won't be offering his neck to anybody anymore, especially after Erza's speech. Besides, he only needs to escape, which should be a breeze for him. It's not like he would fight anybody but dark guild members.