Expires within six months

Expires within six months
 

"My license expires within six months."
"Mi licensia se vence dentro de seis meses."

Is "dentro de" the preferred way to say it?

Yes; also (although more colloquial) "mi licencia se vence en seis meses". :)

Could you use caducar instead of vencer in this case?

:thinking: Hmmm... I wouldn't. We use "caducar" more for foods or substances that get spoiled over time.

- Este medicamento caduca en mayo.
- No me había dado cuenta de que la pasta de dientes ya está caduca.
- No uses esa pimienta; ya caducó.