Every arena has an hours-online limit — you can only play a set number of hours of real-world time within any rolling 24-hour window. The cap is 16 hours per rolling 24h on all arenas.
Ferion can become very intense, and some players break their sleep cycles to take care of their empire. That isn't healthy. The hours cap also closes the gap between players who can pour unlimited time into the game and those with other commitments.
With a 16-hour limit, across any rolling 24-hour window you can only be active for 16 of those 24 wall-clock hours. The window slides with the current moment — at 08:00 today, it covers the period from 08:00 yesterday onward.
You can see your current usage by clicking on this icon at the bottom bar:

The modal shows how many hours you have left, when you used them, and roughly when new hours will free up.

Any action that changes game state marks the current hour as used — for example: moving ships, queuing buildings, changing production, giving a spy an order, accepting diplomacy, etc.
Passive browsing — looking at the map, opening planet panels, reading the leaderboard — costs nothing. Chat also doesn't count.
Activity is bucketed by clock hour, starting on the minute :00. So if you take an action at 08:59 and another at 09:05, you've used two hours (the 08:00 hour and the 09:00 hour), even though only six minutes of real time elapsed. The same hour-bucket can only be charged once — a 1-minute session and a 60-minute session in the same clock hour both cost 1.
This page in other languages: Български: Лимит на часовете онлайн · Čeština: Limit hodin online · Deutsch: Stunden-Online-Limit · Español: Limitación de horas en línea · Français: Limitation d'heures en ligne · Italiano: Limitazione Ore Online · Magyar: Online órák korlátja · Nederlands: Beperking uren online · Polski: Limit godzin online · Português (Brasil): Limitação de Horas Online · Română: Limitarea orelor online · Русский: Лимит часов онлайн · Türkçe: Çevrimiçi Saat Sınırlaması · Українська: Обмеження годин онлайн · 简体中文: 在线时长限制 · 한국어: 온라인 시간 제한