Best way to do analysis of data

Question

I have 18,000 or so records, each marked with a date field, and each recording the elapsed time it took to perform some action, in seconds, as of a particular hour/minute/second.
For example:
TESTTIME         TESTDURATION
7:03:21          2.4
7:03:32          3.1
7:04:01          1.9...and so on for a long time!
I would like to produce a report which says 'in this 5 (or 10 or 15) minute interval, the average time taken to perform the task was X seconds', and to project that out over the entire duration of the test, so that a series of 5 minute (say) averages is produced covering the whole test period.
So far, I have got this:
select
count(*), avg(testduration) from testtable
where testtime>=(select min(testtime) from testtable)
and testtime<=(select min(testtime) from testtable)+5/1440
...and it works, but of course it will only output one average for one time period and to get the next one, I have to re-type quite a lot of the subqueries.
So then I put it all in a PL/SQL loop, but that seems inefficient, especially because I bet there is some sort of analytical function that could do it all in one pass in plain SQL.
Sadly I don't have a clue about analytical functions, so if someone here does and doesn't mind offering a clue, I'd be very grateful.
Of course, if it turns out some other way entirely will produce the desired result, I'd be keen to see that too.

Answer 1

I think you've missed the essential point: that there are (or I think there are) SQL constructs which will produce the report I want in one pass.
Analytical functions, I think they're called. But I don't know which would do it, and I wouldn't have the first idea how to use it.
Besides, the one pass feature (that is, one SQL statement, not repeated re-issues of the same statement with modified WHERE clauses) is another important point that fiddling with truncated dates and substr constructs won't deal with, either, I think.
I've done it in a PL/SQL loop which forever increments the start and end times for the sample (and like, I do have a clue about AVG and SUM). I'd like to know how to do it more efficiently in a single SQL statement.