'Three years on a rock' — sit long enough and even stone warms up.

What a Japanese person actually thinks
Homely and slightly stern — the sort of thing an older relative says about your first job. Genuinely well loved, but it reads as advice rather than as identity.

Meaning

Literally: on a stone + even + three years
Actually: Persist at something long enough and it will eventually pay off. The image is of sitting on a cold rock until your body heat warms it.

Scores

Naturalness●●●●●

Very well-known proverb.

Coolness●●●●●

Seven characters with kana in the middle. Same length problem as most proverbs.

Watch out

Nothing embarrassing lurking in here.

Get this instead

忍耐 nintai
perseverance

Two characters saying what the proverb takes seven to say.
百折不撓 hyakusetsu futō
broken a hundred times, still unbent

The four-character version, with far more force.

On skin

石の上にも三年

Orientation: vertical

Mincho, vertical.