It depends on the individual, their skills, and their work experience as follows:
Beginner: 2–4 Years (Full Time- Part-time)
Intermediate: 6-18 Months (If you are already into IT and have basic coding skills)
Experienced: 2–6 Months to map some additional skills and to become an expert, Full Stack developer.
If you want to become a full-stack developer, check out this Full stack Developer Course by Intellipaat.
Also, watch this video tutorial: