Spawn enemy intervel

[fasterthanlight 10]

How can I make a certain object (marker) create an opfor every 15 minutes?

[demonized 20]

use a while loop on getMarkerPos "yourmarker".

while {true} do {
create the unit here
sleep (60 * 15);
};

be careful with the max amount of groups for that side, 144, though it will take you 36 hours to reach that just with this loop.

[igneous01 19]

maybe you should have a look at this script: http://forums.bistudio.com/showthread.php?t=117220

does what you want it to do and more.