The rule in English is to form the plural by adding "s" (epochs)
or sometimes "es" (churches). Sometimes the stem mutates in a
regular way (stories). A few plurals are spectacularly irregular
(children). A reasonable discussion can be found at http://www.bartleby.com/64/81.html
There is never any good reason to use apostrophe-s when forming
the plural.
Doing so is a common mistake, but it remains a mistake. It is
called the "grocer's apostrophe". It is the epitome of uncouthness.
If I were a grocer I'd be offended.
Some editors will tolerate apostrophes in the following unusual
cases, although in every case there is a better alternative:
*) After figures: /some/ editors will tolerate
the 1800's and the 1900's
However, those don't forms really look any better than
the 1800s and the 1900s.
*) After letters: /some/ editors will tolerate
Dotting the t's and crossing the i's.
That's ugly, but doing without the apostrophy is ugly, too:
Dotting the ts and crossing the is.
I would prefer to solve the problem with italics:
Dotting the /t/s and crossing the /i/s.
... or if italics are not available, with quotation marks:
Dotting the "t"s and crossing the "i"s.
*) After upper-case letters in buzzwords and acronyms, the regular
plural works just fine:
The GIs saw the UFOs.