There is no rule that covers all abbreviations, and if in doubt, it's a good idea to consult a dictionary to find the generally accepted pronunciation.
Since the abbreviations you mention all come from Latin, I'll mention a few guidelines for those. In general, if the abbreviation is two letters separated by a period, the individual letters are pronounced:
i.e. is pronounced eye - yee
e.g. is pronounced yee - gee
These are formally known as initialisms, but that is of little help, since what makes an initialism an initialism is pronouncing the individual letters separately.
On the other hand, where a syllable has been borrowed from Latin, the full Latin phrase is pronounced:
etc. is pronounced et cetera
These are not firm rules even for Latin and exceptions do exist, such as pp. which is not an initialism and is pronounced pages.
When we're talking about abbreviations that don't come directly from Latin, it's a bit harder. For example, we have
Mr. which is pronounced mister
CEO which is pronounced cee - yee - oh
NASA which is pronounced naa - saa
There are a few hints such as capitalization that can guide you as to the proper pronunciation, but English is riddled with exceptions, and the best way to learn this sort of thing is to (1) listen to English spoken by a variety of native speakers, and (2) invest in a good dictionary that you can look up pronunciations in.
Best Answer
Practice varies from publisher to publisher, but these are common abbreviations: