INTERVAL in a function

I have a simple function which I use to set up a users' expiry date. If
a field in a table contains an interval then
this function returns a timestamp some time in the future (usually two
weeks), null otherwise. I can't pass the
interval from the table into a variable properly within the function.
Any ideas?

CREATE OR REPLACE FUNCTION getUnitTimeLength(int) RETURNS timestamp AS '

DECLARE
grpID ALIAS FOR $1;
intval INTERVAL;
exptime TIMESTAMP;
BEGIN
SELECT INTO intval unitTimeLength::INTERVAL FROM customer.groups WHERE groupsID = grpID;
IF intval IS NULL THEN
RETURN NULL;
ELSE
SELECT INTO exptime current_timestamp + INTERVAL ''intval'';
RETURN exptime;
END IF;
END;
' LANGUAGE 'plpgsql';

SELECT getUnitTimeLength(55);

ERROR: invalid input syntax for type interval: "intval"
CONTEXT: PL/pgSQL function "getunittimelength" line 11 at select into variables

However if I change the else clause to this:
ELSE
SELECT INTO exptime current_timestamp;
RETURN exptime;
END IF;
it works:
----------------------------
2004-11-08 16:14:40.273597
(1 row)

Thanks
Ron