Each titled section in Familia Rōmāna is divided up into subsections by tiny roman numerals (I, II, III) in the margin. Each of these subsections is designed to be one lesson. I can't remember exactly how many there are in total but on a rough estimate the book consists of roughly one hundred such subsections. Now, I think it is reasonable to devote about an hour to teaching each lesson (some lessons might take longer, sometimes less). So I think one can reasonably expect to teach the whole book in 100 hours.
But I'm always somewhat surprised by people who use the book as a primary resource to learn Latin through self-study. I've always understood it as a textbook for the 'Living Latin' or 'Spoken Latin' approach. One can use it as a basis to teach Latin exclusively through the medium of the Latin language, and in principle by the end of it (or after a few chapters of the successor book Rōma Aeterna) the student is in a position to start reading real texts. But in such classes, the point is not just to read the text aloud; the teacher must also constantly pose questions to the students based on the text to ensure they have understood it and to get them to speak using the phrases they learn, to say nothing of explaining difficulties, adding additional exercises and activities, gently correcting the students' errors, etc.
I suppose one can just try reading and understanding it by oneself, but I'm a little doubtful of whether this would result in a reasonable understanding of the grammar by itself (the question is: can one ever learn a language purely by reading? and even if so, does the book provide enough text for that?). Or one might try doing other things with it; I once heard of someone laboriously going through translating it into English (although this seems to me rather to defeat the original point of the book's existence; in that case why not use a traditional primer with translation exercises and a key?). One might supplement it with a textbook that explains the grammar more explicitly (but then, one might equally use a different textbook in a modern language). Still, if a method works for someone, why not?
Mind you, it is true that the linguistic experience you already have makes a big difference. If you are familiar with linguistics and other foreign languages, then one might just skim through a straightforward grammar of the language (like Kennedy's) to familiarise yourself with the rules, make a few notes about anything particularly difficult, and then start reading. I have tried this myself with various languages (although with somewhat mixed success). I'm not sure from your phrasing if you've studied modern or ancient Greek, but if you've learned ancient Greek to a good level, then learning Latin is a fairly simple matter (because the grammar is so similar), and just reading through FR carefully might well be a fairly good way to learn. Even if it's modern Greek, the combination of languages you speak of above should give you a good grounding in grammatical concepts. In short, I wouldn't be surprised if you picked up the language much more quickly than others, no.
