And when do I know the apropriate time to get it?

I have formed an alliance with the Syreen and told the Mycon about the planet that's perfect for their deep children. When They are away and hopefully being ambushed by the Syreen my task is to get their sun device from the planet with just a few guards left. The problem is that there seems to still be a lot of them on that planet. Maybe even infinite?? I have tried killing a bunch of them but apparently not enough.

I don't have the gog version of the game but The Free community modified version The Ur-Quan Masters. http://sc2.sourceforge.net/

This is what the the wiki Ultronomicon says:
The Sun Device is located on Beta Brahe I (639.5 : 231.2), but it is guarded by a large (infinite, in fact) fleet of Mycon Podships. To acquire it, follow the steps to form an alliance with the Syreen, and either visit Beta Brahe I while they are away or after they come back. If you wait for them to come back, you still have to fight them, but the fleet deployed at Beta Brahe I is finite, and can be destroyed so that you can pick up the Sun Device.
http://wiki.uqm.stack.nl/Walkthrough#Sun_Device

Some other guide I found on the internet said 5 ships where left guarding the planet.

Edit:
Solved it. Had to talk to the Mycon homeworld at Epsilon Scorpii, not Beta Brahe where the sun device is. And apparently I have to fight 5 Mycon ships at Beta Brahe.