system
Given the following post, respond as if you're commenting on Reddit:
human
[Warhammer 40k] How fast do Orks reproduce and how long does it take for them to double in population size?
gpt
Fast. Greenskins reproduce asexually. When a greenskin is killed, they release a burst of spores.

These spores grow into the entire greenskin ecosystem. Some become mushrooms and fungus. Others grow into snotlings, various types of squigs, gretchin and ofcourse orks. This means that killing orks is only a temporary solution because all it does is infest the land with more greenskins.

It also means that orks will thrive, even in the most hostile environments because they bring their own ecosystem. A harsh land might kill a lot of orks at first. But pretty soon there will be mushroom patches that feed the smaller squigs. Bigger squigs that feed on the smaller squigs and orks that feed on the bigger squigs.

Most warzones that were the site of a major waaagh being destroyed suffer from permanent ork infestations, forcing the local planetary defense force to go on frequent extermination missions to keep the population down.

It's also not uncommon for planets that that suffer a second or third waaagh invasion to suddenly find themselves flanked by tribes of feral orks that appear from the deep jungles or other hostile areas of the planet to support the main horde. These tribes had been living in seclusion until a large waaagh arrived and they felt the call of the horde.

Seemingly solid fortifications are suddenly overrun by feral orks charging the rear of the lines with stone weapons.

I don't really have a time path for you but sufficient to say that ork populations grow fast and they're very hard to get rid of. Another problem is that orks grow bigger and tougher with conflict. So any failed efforts to exterminate them tend to make them nastier.