SME = Subject Matter Expert. Companies want people that are SMEs (in general). With compressed schedules and ever increasing smart end-users the demand strips the supply.
How Long does it Take?
Over 20 years ago, an office mate and I discussed this very topic. Our environment was constantly churning in new people as others left. We became the mentors for the new folks.
The code was complex as is any current medium to large web site is today. We both agreed it takes a full year to kind-of sort get comfortable and at least another 6 months to enter into the start of expert mode.
Nothing has changed in all those years, except that the complexity is even higher now. Learning never ceases in this industry. Work hard and diligent and things will get better.