Did you see:
http://www.postgresql.org/users-lounge/docs/7.0/user/functions2872.htm
On Mon, 5 Mar 2001, Josh Berkus wrote:
> I can't find any documentation for what masks to use with the function
> TO_CHAR(INTERVAL, mask). Is there a TO_CHAR(INTERVAL)? If so, what
> masks are there? If not, how would you suggest I convert an interval
> value for user-friendly display?