It's not really a case that there were a 'bunch of rules' to 'master'; instead, Classical Rhetoric as found in Greece and Rome was a highly developed art form, which had several components, involved the bulk of their form of secondary and tertiary education, and played a key role in society.

Rhetoric as a distinct 'subject' which was discussed and analysed appears around the 5th century BC, in Athens and contemporary Greek city-states. Although there was no doubt public speaking before then, the nature of Greek democracies and law courts saw a lot of public speaking by engaged citizens, and around this time one sees the first documents that begin to develop a theory of public speaking, features of technical vocabulary, and an attempt to describe, analyse, and recommend methods, for better public speaking.

The role of public speaking changed over time across the ancient world, depending in part on how and where citizens gave public speeches; for example the loss of genuine democracy in Greek city states saw greated attention given to legal speeches; the rise of the Roman principate saw a similar decline in political speeches in Rome, but new forms of oratory emerged based on public occasions and giving honours.

Students of rhetoric studied and sought to master, generally speaking, 5 separate areas:

1. Invention - the analysis of arguments to identify exactly what the point of contention is and how to use a case to persuade on that point.
2. Arrangement - the organisation of a speech into an appropriate sequence, according to the type of speech given.
3. Style - the speaker should give attention to word choice, composition, correctness, ornamentation, etc..
4. Memory - orators were also taught techniques of memorisation in order to commit a speech word-for-word and be able to recite it.
5. Delivery - speakers were trained in control of voice and gesture as to maximise effect of their speeches.

I would not presume to give a full 'account' of rhetorical 'rules', we are talking about several hundred years of varying documents with different philosophies of speechcraft. I would recommend reading Cicero's On the Orator for an introduction to rhetoric in Rome at its height, and if you have a lot of time on your hand, you can tackle Quintilian's Institutio Oratoria for a full-blown study of the topic. For secondary sources, it is very difficult to overlook Kennedy's A new history of classical rhetoric.

I don't think one could say 'how long' it took to become a master. How long does it take to became a masterful public speaker? However the fundamental education period for young men training in rhetoric was from 15 on til 16 or 17. This came after their study in language, grammar, and literature, which ran from age 11 through to 15.

Very happy to answer some follow up questions here.