Passing varchar parameter to INTERVAL

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;