org-agenda-timerange-leaders

org-agenda-timerange-leaders is a variable defined in `org-agenda.el'.
Its value is

("" "(%d/%d): ")

Documentation:
Text preceding timerange entries in the agenda view.
This is a list with two strings. The first applies when the range
is entirely on one day. The second applies if the range spans several days.
The strings may have two "%d" format specifiers which will be filled
with the sequence number of the days, and the total number of days in the
range, respectively.

You can customize this variable.