How long does it take to become a blockchain developer with no experience if you sit and study 6 hours a day?
There's no definitive answer, since each people have different learning speed and each company have different requirement or expectation when hiring a blockchain developer.
The Brink fellowship is an intensive one-year program to onboard exceptional software engineers into Bitcoin protocol development.
But looking at Brink paid fellowship (quoted above), they need a year to do that.