When did a user changed his password in Oracle?

The table sys.user$ contains the field ptime, which keeps the time when the password was changed the last time over. Do not confound it with ctime, which is the "creation time", nor with ltime, which is the time the account has been locked (if any).

SELECT NAME, ptime AS "LAST TIME CHANGED", ctime "CREATION TIME", ltime "LOCKED"

FROM USER$

WHERE ptime IS NOT NULL

ORDER BY ptime DESC;