From Mechanics in Guide part of ABC of Plain Words by Sir E Gowers (1951)

The only functions of the apostrophe that call for notice are (a) its use to denote the possessive of names ending in s and of pronouns, (b) its use before a final s to show that the s is forming the plural of a word or symbol not ordinarily admitting of a plural, and (c) its use in defining periods of time.

(a) There is no universally accepted code of rules governing the formation of the possessive case of words ending in s, but the most favoured practice seems to be not just to put an apostrophe at the end of the word, as one does with an ordinary plural (strangers' gallery), but to add another s — Mr. Jones's room, St. James's street, not Mr. Jones' room, St. James' street.

As to pronouns, all these except the pronoun one dispense with an apostrophe in their possessive cases—hers, yours, theirs, ours and its, but one's, not ones. It's is not the possessive of it but a contraction of it is.

(b) Whether an apostrophe should be used to denote the plural of a word or symbol that does not ordinarily make a plural depends on whether the plural is readily recognisable as such. Unless the reader really needs help it should not be thrust upon him. It is clearly justified with single letters: "there are two o's in woolly"; "mind your p's and q's ". Otherwise it is rarely called for. It should not be used with contractions (e.g. M.P.s) or merely because what is put into the plural is not a noun. Editors of Shakespeare do without it in "Tellest thou me of ifs" and Rudyard Kipling did not think it necessary in:

One million Hows, two million Wheres,
And seven million Whys.

(c) Whether one should use an apostrophe or not in such expressions as "Ten years imprisonment" is a disputed and not very important point. Each can be justified: Ten years' on the ground that the phrase means "imprisonment of ten years", and so years must be in the possessive case; ten years on the ground that it is an adjectival phrase. The ordinary practice seems to be to use the apostrophe for the singular ("a week's interval") but not for the plural ("a six weeks interval").