It takes a lot of time to learn everything: typing effectiveness, base speeds (and roughly the rest of the other base stats of pokemons), abilities, different strategies, kind of teambuilding, working def or offensive cores, getting a good ideas of how synergy works in a comp team, and at the same time keep up with pokemmo's metagame which is constantly evolving.
But the longest part of the learning curve is to learn to have a rough idea of damages.