Time differences between rows, not columns?

Using postgresSQL 8.0 for windows, running on windows 2003 server 16gb
RAM, 3Ghz dual p4.
Language: Java - JDBC postgres driver: postgresql-8.0-310.jdbc3.jar

I have an application with 3 tables (in this context that is)

Table bills

bill_id NOT NULL serial
... And other columns

Table bill_status

bill_id (references bills.bill_id)
statusid int4 (references bill_statuslookup.statusid)
statustime datetime

Table bill_statuslookup

statusid serial not null
statusname varchar(255)

The application basically tracks a workflow of bills flowing fromone
department to another.
Everytime the bill moves one step, an entry is made into the bill_status
table.

Eg.
Bills table
-------------
Bill_id otherfield1

1 ........

Bill_status table:
---------------------
Bill_id statusid statustime

1 10 2005-04-04 00:34:31
1 20 2005-04-05 00:55:00

Bill_statuslookup table:
-------------------------
Statusid Statusname

10 submitted
20 received
30 rejected
40 accepted
..
..

Now my problem is this:

1. Find the time taken for each bill to reach from status 10 to status
20 , given the time of status 10 should be between t1 and t2.

Eg I want to know how much time it took for a bill to be accepted after
it was submitted (criteria: submitted between yesterday and today)

2. I want to know how many bills took <7 days, how many tok 7-15 days,
how many took >15 days etc.

The status is a lookup table because the workflow constantly changes,
and I can't have submitted_on, recd_on accepted_on times etc in the main
bills table as columns because there are way too many statuses in the
life of a bill (read invoice).

Hope its clear as to what I'm looking for.
Any help is greatly appreciated!!

Regards,
Amit