Sometimes one can look at the real world to get some form of idea of how much a person would be able to do (even if it is without the aid of aether and echo). Take knights for example.
By around 20-23 years old, they were knighted and had learned swordplay, jousting (lances), horseback riding, archery, courtly manners (whether they'd follow them is another matter), hawking (this was a common hobby among pages, I'm told by history books), hunting and a couple of other things.
Of course, they started as pages around age 7, but it should give some idea.
Some Shaolin monks start training at age 3 and are considered good enough by 11 to demonstrate their style for the public as an example.
Disciplines of Magic is a bit harder to find an equivalent for, but arcanists could be considered magical mathematicians, and if they immersed themselves like monks and knights in their education, it would probably take about 4-5 years to master, if not less (remember, today our education is spread all over our teachers' schedules of many students, if there was only one class to concentrate on, and every sunlit hour of every day was math hour, lots of ground would be covered much more quickly).
Thaumaturgy and Conjury seems to be about the same but opposite ends of the same education (Conjury teaches to mend and heal with external power, while Thaumaturgy teaches destructive power from within oneself), they both also seem to have some ecclesiastical purpose. I'm thinking around 5-6 years for that (again, could be less), considering that they need to learn application of their powers as well (assess injuries/corruption, control of destructive power) on top of the ecclesiastical duties.
Now, if we were to add all this up, we'd get around 30 years of education to have mastered all the classes to a point, and that only if there is no cross-class perks (conditioned body from knight, balance from monk, reasoning and logic from arcanist, elemental knowledge, tuned to aether from magic and so on. Do we even know if echo works on math? Because that could help with arcanima).
The problem isn't really a character that is trained in a lot of disciplines. It is a character that's mastered all the disciplines, have a developed social network outside the disciples, an rich adventuring history and other time consuming things that would get in the way of learning those disciplines.
And even if they're trained in all the disciplines until mastery? There is knowledge, and then there's experience. A career student of a monk will have a harder time making their punches land in the optimal area even if they know where they should punch, than a monk who have fought many enemies and know how to compensate for enemy movement in life-or-death situations.
So for me, it isn't unrealistic that a multiclass character would exist. It would only be unrealistic if they know what to do in every situation, never make a mistake and just generally be perfect before age 50 (we must allow for them to learn how to walk, talk and eat first, right?
 And then they study, and then they experience the real world). Also, in-universe, you'd likely not be considered an expert without a lifetime's study in the relevant areas.
Of course, this is all assuming a character who was in the position of being a full time student.
As and aside: DoL and DoH would take a lot longer to learn and master than DoW and DoM, judging by the real world equivalents.
By around 20-23 years old, they were knighted and had learned swordplay, jousting (lances), horseback riding, archery, courtly manners (whether they'd follow them is another matter), hawking (this was a common hobby among pages, I'm told by history books), hunting and a couple of other things.
Of course, they started as pages around age 7, but it should give some idea.
Some Shaolin monks start training at age 3 and are considered good enough by 11 to demonstrate their style for the public as an example.
Disciplines of Magic is a bit harder to find an equivalent for, but arcanists could be considered magical mathematicians, and if they immersed themselves like monks and knights in their education, it would probably take about 4-5 years to master, if not less (remember, today our education is spread all over our teachers' schedules of many students, if there was only one class to concentrate on, and every sunlit hour of every day was math hour, lots of ground would be covered much more quickly).
Thaumaturgy and Conjury seems to be about the same but opposite ends of the same education (Conjury teaches to mend and heal with external power, while Thaumaturgy teaches destructive power from within oneself), they both also seem to have some ecclesiastical purpose. I'm thinking around 5-6 years for that (again, could be less), considering that they need to learn application of their powers as well (assess injuries/corruption, control of destructive power) on top of the ecclesiastical duties.
Now, if we were to add all this up, we'd get around 30 years of education to have mastered all the classes to a point, and that only if there is no cross-class perks (conditioned body from knight, balance from monk, reasoning and logic from arcanist, elemental knowledge, tuned to aether from magic and so on. Do we even know if echo works on math? Because that could help with arcanima).
The problem isn't really a character that is trained in a lot of disciplines. It is a character that's mastered all the disciplines, have a developed social network outside the disciples, an rich adventuring history and other time consuming things that would get in the way of learning those disciplines.
And even if they're trained in all the disciplines until mastery? There is knowledge, and then there's experience. A career student of a monk will have a harder time making their punches land in the optimal area even if they know where they should punch, than a monk who have fought many enemies and know how to compensate for enemy movement in life-or-death situations.
So for me, it isn't unrealistic that a multiclass character would exist. It would only be unrealistic if they know what to do in every situation, never make a mistake and just generally be perfect before age 50 (we must allow for them to learn how to walk, talk and eat first, right?
 And then they study, and then they experience the real world). Also, in-universe, you'd likely not be considered an expert without a lifetime's study in the relevant areas.Of course, this is all assuming a character who was in the position of being a full time student.
As and aside: DoL and DoH would take a lot longer to learn and master than DoW and DoM, judging by the real world equivalents.
