Just what it says on the tin, really ─ how fast do sound changes happen? Since sound changes are generally individual rules, how many of them would occur in, say, a 100-year range? How much variation is there?
Sound changes take place in just a few years, but grammatical changes regarding sound (in a broad sense) take longer. However, they can be generational, in the sense that Smith's grammar can be significantly different from Abu Smith's grammar. On the third hand, if you mean "changes in grammar that propagate to all speakers of the language", then there is rarely any sound change, instead there is language change – one group of speakers does not change their grammar, and the language splits into two (we call them "dialects" for a while, then more differences accumulate and we call them different "languages").