It differs between different national aviation authorities.
For civil ATC in the UK, it's nominally max 2 hrs on any operational position and 30 min rest for every 2 hours operational, however some very busy units mandate less than 2 hours on position, whilst other less busy units may be permitted to extend to up to 4 hours, but even so they must still have 30 min rest for every 2 hours on duty, with a maximum operational duty time of 10 hours and a minimum of 12 hours between shifts..