from galaxy screen click on a system ( one that isn't full )
In system screen on the left hand side above the approval percentage there is a population number, hover mouse over that number.

This shows the number of turns for population loss / gain assuming nothing else happens.

Thank you for your precise answer.
So we need at least 40 food production before colonizing another planet. Okay?

So when your colonizer sacrifices itself, it gets food shipments from your highest producing system ( usually home world ), you can change the systems it receives food shipments from. In essence your trading growth from established system, in order too turn a colony into a system faster.

There is nothing stopping you from spamming colonizer and starting 10 colony when you only have a home system. The home system will just go into super negative and lose a population a turn.

Originally posted by louasphalappe:

When you have negative food (-3 for example), how many turns does it take to lose a population?

When a colony is created, 300 units of Food are required. Hence, if -10 food on a donor system and one population unit disappears, in about 30 turns. This applies to all factions except Riftborn. I can't say for sure about Vodiani and the Umbral Choir. You can choose which colony supplies the outpost with food.

Originally posted by louasphalappe:

Thank you for your precise answer.
So we need at least 40 food production before colonizing another planet. Okay?

A colony needs 300 units of food. You can spend Dust to speed up colony creation, bring the needed food for Influence. A United Empire has a quest, one of the rewards will be increased food growth at the outpost.

Riftborn population mechanic is different, they don't ship food. Riftborn population doesnt naturally grow. Riftborn population have too be built in production que.

Vodyani use essence too create arks. Once arks are created they can be used for war, or you can anchor them too a system and build systems improvements inside the arks.

Umbral Choir only have 1 system. They have sanctuaries

https://endless-space-2.fandom.com/wiki/The_Umbral_Choir/Gameplay