How much time you need to prepare depends on your background and current familiarity with DSA. If I were you, here's what I would do:

- Meta is known for using DSA problems that are [tagged on Leetcode](https://leetcode.com/company/facebook). Pick \~5 recent problems and try to solve each in 20 minutes (Remember, Meta tends to ask 2 DSA questions per interview block).
- If you struggle to solve those problems in the right period of time, schedule the interview further out (3-4 weeks).
- Now the question is -- how to study during those weeks?
  - If you struggled to understand the solutions to those Leetcode problems, go through a crash course like [Neetcode's roadmap](https://leetcode.com/company/facebook).
  - If you understood the solutions but just couldn't get to them fast enough, keep going through the list of tagged questions on LC until your speed improves

Hope that helps, and best of luck