Passing varchar parameter to INTERVAL

Good afternoon,

when trying to create a custom function to temporary ban a user:

CREATE OR REPLACE FUNCTION words_ban_user(
IN in_uid integer,
IN in_until varchar, -- '1 week' OR '1 month' OR '1 year'
IN in_reason varchar)
RETURNS void AS
$func$
BEGIN
........
UPDATE words_users SET
banned_until = CURRENT_TIMESTAMP + INTERVAL in_until,
banned_reason = in_reason,
vip_until = vip_until + INTERVAL in_until, -- for
paying user
grand_until = grand_until + INTERVAL in_until
WHERE uid = in_uid;

END
$func$ LANGUAGE plpgsql;

in 9.5.4 I unfortunately get the error:

ERROR: syntax error at or near "in_until"
LINE 69: ... banned_until = CURRENT_TIMESTAMP + INTERVAL in_until,
^
Is there please a better way here?

Thank you
Alex