For me, it took 6 weeks to learn the basics of R Programming required for Data Science. First two weeks to learn the fundamentals of R programming such as data types, operators, variables, and functions, loops, and conditional statements. Third and fourth week to learn creating, appending, subsetting datasets, and to analyze the data and visualization. Then, fifth and sixth weeks to get familiar with statistical models like linear regression, logistic regression, and implementing using R.
I suggest this R for Data Science course by Intellipaat if you are interested to learn R from top experts.
You can watch this video on Introduction to Data Science with R to get a quick start: