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