British Standard BS 5266: Part 1: 2005

Non-maintained - The emergency light units only illuminate in the event of a mains failure (emergency duration 2 hours)

Maintained - The emergency light units are illuminated at all times using the same lamps for both normal and emergency operation (emergency duration 3 hours)

Sustained - The emergency light units are fitted with two lamps or two sets of lamps. One of which operates on mains 240V AC supply, the other which operates from the battery supply in the event of mains failure. It is basically a non-maintained system with the addition of mains lamps which should be illuminated whenever the premises are occupied (emergency duration 1 hour)

As far as i was aware 3 hour lights are installed as standard as it is easier to manufacture 3 hours lights as it is to manufacture 1 hour light units. But you would only require 1 hour light units in office environment for example but where there is sleeping accommodation then 3 hours would be installed.

but as said above everyone tends just to install 3 hours units no matter what the environment or use of the premises.The reason for 3 hours in sleeping accommodation if the electric fails then you don't have to disturb guests but when you are in the last hour you should in theory wake quests because if there was a fire then anything less than 1 hour battery life would be deemed not enough.

Phil