A literal translation of long comme un jour sans pain would be, “as long as a day without bread.” The idiom is used to express that something is very long — in reference to physical length (a long road, a long list) or, more frequently, to the duration of an event (a long speech, a long wait) — and dreary.
Like most idioms having to do with bread, this one dates back to a time when bread was the foundation of the average Frenchman’s diet: if there was no bread to be had, it really meant that there was no food at all. And if you were to spend an entire day without food, then surely that day would feel excruciatingly long. When it originally appeared in the 17th century, this idiom was an expression of length only, either physical or temporal. It’s not until the 18th century that it took on a secondary notion of dullness, based on the idea that what seems unending is also likely to be boring.
Cette réunion était longue comme un jour sans pain.
That meeting was endless and boring.
Cette liste est longue comme un jour sans pain.
And this is one item on a list long like my arm.
Another useful expression using bread is Ça ne mange pas de pain. Literally translated as, “It doesn’t eat bread,” it is used to say that a thing or an action can’t hurt: it may never amount to much or be of much use, but if it costs nothing and entails no risk, then why not?
It is a colloquial expression that is usually delivered with a shrug, and when spoken, the ne and the de are often swallowed, so that you will hear it as, "Ça mange pas d’pain."
This expression also dates back to the 17th century, when bread was the cornerstone of one’s diet, and therefore a symbol of sustenance in general. Consequently, if something didn’t eat any bread, it meant it wouldn’t eat into your supplies or make you any poorer. Then there was no reason to go without it.
On peut toujours demander, ça ne mange pas de pain.
It can never hurt to ask.
Ça ne mange pas de pain d'essayer.
There is no harm in trying.