What happens when this wears off from a planet? Does population automatically regenerate to full or some % thereof?
I want to re-attempt an invasion and the virus wears off before my timer resets for invasion attempt.
Also, any invasion tips from you experts out there? This is only my first try lol.

Make sure your attack is higher than the combined defence+attack of the planet for better chance of invasion

The population immediately returns to maximum once the virus wears off. At least in the cases I've seen, anyway.