There is ho hard value as far as I know, they probably use some sort of formula that takes into consideration your usage during the 07:00-00:00 time period.
They do the nightsurfer thing to encourage users to change their download habits. p2p traffic places heavy loads on a network and they would prefer everybody getting a good experience during normal hours.
You can do a schiteload of downloading during nightsurfer time &voutside that time stream tv/movies at night, do youtube, browsing, music, smaller downloads etc during the day and you will be fine.