org-agenda-time-grid

org-agenda-time-grid is a variable defined in `org-agenda.el'.
Its value is

((daily today require-timed)
 "----------------"
 (800 1000 1200 1400 1600 1800 2000))

Documentation:
The settings for time grid for agenda display.
This is a list of three items. The first item is again a list. It contains
symbols specifying conditions when the grid should be displayed:

daily if the agenda shows a single day
weekly if the agenda shows an entire week
today show grid on current date, independent of daily/weekly display
require-timed show grid only if at least one item has a time specification
remove-match skip grid times already present in an entry

The second item is a string which will be placed behind the grid time.

The third item is a list of integers, indicating the times that should have
a grid line.

You can customize this variable.