'You only live once' — natural Japanese, but seven characters and a magazine-headline register.

What a Japanese person actually thinks
Reads like the cover line on a travel magazine or a life-insurance advert. Sincere but very commercial.

Literally: one time only + life
Actually: 'A life you get only once.' Correct and idiomatic. It's also stock lifestyle-magazine copy.

Naturalness●●●●●

Natural Japanese phrasing.

Coolness●●●●●

Long, mixed script, and the sentiment is a cliché in both languages.

Nothing embarrassing lurking in here.

一期一会 ichigo ichie
every meeting happens only once

The classical Japanese expression of the same instinct, four dense characters, genuine cultural weight.
今を生きる ima wo ikiru
live in the now

If it must be a phrase, this one is shorter and less advertorial.

一度きりの人生

Orientation: vertical

Mincho.