I will try to answer indirectly, cause i am still not an animator. But i can draw lets say, fast,(not pretty fast maybe, but that close). As everything it takes time to become good. And animations frames as you can notice are never full body, so there are not much details as in an illustration generally. I guess that before animating from zero they try put the best drawings first, and i guess they take more time than the others. So for example, if you have a drawing style like love is war it will take for the keyframes maybe 25 to 40 minutes. Inbetweens if you put them it will takes probably 20 or 30 minutes . And this is for a simple styles. If you take a style like kabaneri it takes more probably. More lines , more times. But its not important to fkus on the time. Whats important is to improve the skill first. And if you practice daily you can become very good in 1 year. Cause animation is just a process. What isnt a process is the skill.