Any suggestions to learn how to master the “T” flap? and on the other hand, How long does it take to learn it?
Depends on which accent of English you speak. As an American, more often than not, in spoken speech it is closer to a mix between “d” sound and a “t” sound, unless you’re emphasizing the word, then it would be a stronger “t” sound.
Most of the time with pronunciation, you need to practice it. Try finding sentences with audio that use this “t” flap a lot, listen, repeat, record, listen to your recording, and do that a lot and you will eventually get to where you can master it/pronounce it.
I also don’t think this is a huge deal in English, because I even just asked a few people what word I was saying by replacing the “t” flap with a “d”, and they still understood perfectly fine, and didn’t even notice I was saying it differently.
Here is a quick video with transcript going over the topic.
Es básicamente idéntica a una “r” simple en español, como en la palabra “caro”. De hecho, así se representa en IPA
En definitiva, pronuncia una “r” española corta y ya está. Tiempo: unos pocos segundos