In the first episode she was strong enough to be a seated officer (AKA not fit to lick the boots of Uryu with his power glove) and from then to when Aizen took the Hougyoku out of her she was essentially powerless. She had a few weeks to train from when Ichigo went back to the real world to when Hitsugaya's team came to the real world (ignoring the filler arc), by that point she could kill a weak Arrancar with her releases special ability. Then there was another month of her training with Orihime in SS (as if this would make anyone stronger). So her power level should be first episode level plus six or seven weeks of potential off screen training.

Realistically this shouldn't be enough time to go from shitting one's self in fear of an opponent to being able to one shot them, but it's not really unprecedented in Bleach. I believe Ishida only trained with his new bow for a week or two before the SS arc and he forced a captain to bankai (while significantly weakened due to poisoning), and Chad was able to completely steam roll grunt Shinigami after only a week or two of training with Yoruichi.