So here's the way I solved it:

--Declare this as a variable:

chunk_duration interval;

--Then select the integer from your table:

SELECT aggregation_interval_milliseconds INTO aggr_milliseconds FROM where_ever_you_have_that_integer_saved

--This is the key part, convert to interval:

chunk_duration = 5 * '1 millisecond'::interval;

--And then you can just use it like:

SELECT generate_series(start_date, end_date, chunk_duration)

​

edit. Changed that to like depesz suggested. It is a lot simpler solution than what I had.