A shitload.
Hmmm, nonsensical examples aside, this might be another one of those things that differs depending on reference frame (i.e. whether the Atlantic Ocean is to the East or West of you). “One Hundred Twenty Three and forty-five hundredths” might be the American convention, but it sounds very odd to my ears, certainly isn’t the standard over here.
Needless to say that other non-English speaking nations sometimes do it very differently.
Informally, there’s nothing wrong with it, and we use it frequently: “Doctor, my temp is a hundred and two point eight.” But in more formal contexts, it leaves something to be desired. It’s a little like the difference between “didn’t” and “did not,” or perhaps, “sneaked” and “snuck,” “isn’t” and “ain’t.”
Anyway, if you’re gonna abbreviate, might as well go whole hog:
not “one-hundred and twenty three point four five”
but “one twenty-three point four five” [removing all place value referents save the decimal point]
Most people probably don’t, whatever their age; I don’t (especially not in inches, which I usually divide into powers of 2, not hundredths). But “cents” is in a sense (ha ha ha) equivalent to hundredths, and Americans are less likely to say “three hundred and twenty-five point eight six dollars” or “three hundred and twenty-five dollars point eight six” than “three hundred and twenty-five dollars and eighty-six cents” – or even, to get back to what I was saying above, “three twenty-five eighty-six.” [Note, no place value referents at all.] Basically the same principle. (I see you’re British, so YCMV.)
I don’t like that. What do you get when you multiply 123 by 1000? Is it “one hundred twenty-three thousand” or “one twenty-three thousand”? The latter isn’t ambiguous in any way that I can tell, but do you know anyone who would actually go along with it?
Anyway, if you’re going to go “whole hog” your best bet is “one two three point four five”, no?
Yeah, as I said, usage seems to be changing, but I do know some people who use billion as 10^12, and on continental Europe, it’s pretty common (if not standard) for billion to be 10^12. There’s a word quite common in Europe for 10^9 and that’s “milliard” (which is sometimes used in British English, but far more common in French, Hungarian, etc…) To avoid confusion, most European economic magazines in English I’ve seen do not use the word “billion,” but rather “thousand million” and the such.
As for trillion, quadrillion, etc. In American usage, a trillion is 1,000 billion, a quadrillion is 1,000 trillion, a quintillion is 1,000 quadrillion, etc. In traditional UK and European usage, a trillion is a million billion, a quadillion is a million trillion, a quintillion is a million quadrillion, etc.
That’s why the form “x*10^y” is a less confusing way to say numbers.