The first wave of 20 Men In Black parachutists is not a timed event but determined by the distance they halo and parachute from - any higher and the mission breaks.
The waves of replacements are determined by the following code:
Changing this could allow for a little more time between the groups after killing the majority on the ground... see explanation below.
So if there are less than 5 ai alive on the ground then the server will spawn a replcement wave of 4 parachutists @ 1000 meters above the ground - they halo and then open the chutes then float down and finally reach the ground after about a minute or so ( the halo and parachute phase is not part of the code timer diag_tickTime ) therefore the time between reinforcements is greater than 180 seconds. this means a player must kill the last 5 units within 3 minutes or more spawn above.
This does have an overlap however which is not intended where - when you kill the last 4 the diag_tickTime has already expired and the next 4 ai are already falling.
Lets put it this way:
As soon as you kill 1 of the remaining 5 then the diag_tickTime starts and you now only have 3 minutes to kill the remaining 4 ai - if you succeed then all is good but fail the timer and 4 parachutists are now falling from above but this presents the catch in the mission as there are only 4 ai not 5 so the timer is already skipped / ticked as the total number of alive ai is less than 5, the next group of 4 parachutist is already coming bringing the total to 8 parachutists. Kill 3 of the new 8 and your back to the last 5 and the 3 minute timer barrier.
Effectively you have to kill about 6-8 ai in 5 minutes or the process continues but there is a total of 20 ai in reserve so the mission will complete eventually once you kill the total of 47 ai.