"three, hour-long sessions" vs. "three hour-long sessions"

Solution 1:

I would omit the comma, because if I understand your context correctly, you intend for "three" to modify "hour-long sessions", not for "three" and "hour-long" to modify "sessions" independently.

Perhaps Wikipedia on commas between adjectives will help:

A comma is used to separate coordinate adjectives; that is, adjectives that directly and equally modify the following noun. Adjectives are considered coordinate if the meaning would be the same if their order were reversed or if and were placed between them. For example:

• The dull, incessant droning but the cute little cottage.

• The devious lazy red frog suggests there are lazy red frogs (one of which is devious), while the devious, lazy red frog does not carry this connotation.

If we reverse the order of "three" and "hour-long" in your example, we obtain "hour-long, three sessions", which doesn't make sense. If we add and between "three" and "hour-long", we obtain "three and hour-long sessions", which again doesn't make sense. This implies that "three" and "hour-long" are not coordinate adjectives and should not be separated by a comma. Without the comma, "three" modifies "hour-long sessions", so "three hour-long sessions" means three sessions, each one hour long.

Solution 2:

Grammar School

Prepare for a big surprise. These are all grammatical:

1. The interview is broken up into three hour long sessions.
2. The interview is broken up into three hour-long sessions.
3. The interview is broken up into three-hour-long sessions.
4. The interview is broken up into three-hour long-sessions.
5. the Interview is broken up into three hour long sessions.
6. the intervue iss broken up, into, three, hour, lawng, sesssions.

They are all grammatical, and some of them mean different things. And some of them are poorly written. But the grammar doesn’t usually care about bad writing technology.

That’s because grammar is unaffected by spelling, punctuation, capitalization, spacing and indentation, and all the rest. Most of those fall into the category we call orthography — that is, they’re about writing not grammar.

Grammar comprises the subfields of morphology (how words are put together) and syntax (how sentences are put together).

So it turns out that you’re just talking about style preferences using the inventive technology we call writing, not about grammar at all. However, punctuation can reflect grammar, so if you use different punctuation, you may be sending signals that the grammar is different.

Sometimes.

What’s the difference again?

All that said, most readers will interpret

• broken up into three-hour-long sessions

and

• broken up into three hour-long sessions

differently, using punctuation to represent nuances in the spoken language that convey the differences in the underlying grammar between those two choices.

There may even be an argument to be made that

• broken up into three-hour long-sessions

could be its own thing, too, at least in a rescue reading.

But how should I write it?

This comes down to a matter of style, but the simplest and most natural way to punctuate your sentence to let readers mean what it is I think that you mean is this way:

The interview is broken up into three hour-long sessions.

No commas needed there, and the hyphen is meant to convey grouping and intonation. These allow readers to infer your sentence’s actual grammar, at least readers who agree on what your writing means.

If you worry that your readers may not be as sensitive to these things as you are, or might interpret your technology differently than you do, then you could consider changing its wording, something which now at last does alter its grammar:

The interview is broken up into sessions, each an hour long.

There are many other possible rewrites, of course.