In order to draw a schematic that works, you need to fully understand any and all hardware requirements for every single pin of every single component including the microcontroller. You also need to make sure that everything you draw complies with these requirements. This sounds daunting, but for the vast majority of them it will be very simple.
There is a huge amount to master in the process you have described, but you do not necessarily need to master everything in order to get a working design. This is because there is more than one definition of working. For example, there are many mistakes you can make with a layout that do not matter if you do not need your device to comply with EMC regulations, or if your ADC noise and accuracy requirements are relaxed, or if you do not require high speed signalling.
The same is true of the schematic design. Do you care about performance over temperature? Protection from surges? Protection from users plugging in the wrong power source or signaling cables? If your definition of "working" is very relaxed you can tolerate many errors.
I suggest you buy an eval board for the micro you want to use, as well as a solderless breadboard and some compatible components and have a go at putting the circuit together. study the schematic for the eval kit (and eventually copy it). Test everything you do using the hardware before drawing it in the schematic. Make sure what you draw is exactly the same as your breadboard prototype. Dont ignore the power supply or jtag connections - either copy them exactly from the dev kit or test your implementation before committing it to your design.
Have fun!
Edit: you have specifically asked how long it will take to learn to do this. This is an unanswerable question! If your design is simple, I would expect a good EE graduate from a degree which has elements of this work in the degree programme to be able to do this in a month or 2 (to commercial standards, with guidance from seniors). If the requirements are complex further training may be required (up to 6 years?). If only a hobbyist level of working is required, a person with tinkering experience could learn this in less than 6 months maybe? So the answer is between a few months and 10 years.
