A lifetime, and honestly, no one - contrary to the current trend in job titles - will ever be a true full-stack developer these days.
In the early 90's a full-stack developer was super-rare to find and the title was left to anyone who was able to master - not just one stack, like the current MEAN stack for instance, but - every single stack available. From Fortran to Java to PHP to HTML to JavaScript to bash to every type of server, database and programming language known to man. I believe there were about two people that fit that bill up to around 1992-1994.
Now they should be considered individual stacks like LAMP stack, MERN stack, etc.
That, however, is my personal opinion based on over 20 years of experience.
My advice would be to focus on learning a single programming language. You will find that it will snowball into learning other programming languages, frameworks, resources, etc.
It's more about mastering your own work ethics and dedication to the art of what you're learning, and when you've reached an advanced level of one or more of those skills - it's most likely because you've realized that anything we become great at, can always be refined and made greater.