Buddha Sasana of Ariya Maitreya Buddha The Sasana of Gautama Buddha will last for 5000 years of which more than 2500 years are over. Ariya Maitreya will be the next Buddha. How long will the Sasana of Ariya Maitreya Buddha last?
I am a Theravada Buddhist and as far as predictions go here's what i was able to find...


  The coming of Maitreya will be characterized by a number of physical
  events. The oceans are predicted to decrease in size, allowing
  Maitreya to traverse them freely. Maitreya will then reintroduce true
  dharma to the world.
  
  His arrival will signify the end of the middle time, the time between
  the fourth Buddha, Gautama Buddha, and the fifth Buddha, Maitreya,
  which is viewed as a low point of human existence. According to the
  Cakkavatti Sutta: The Wheel-turning Emperor, Digha Nikaya 26 of the
  Sutta Pitaka of the Pāli Canon), Maitreya Buddha will be born in a
  time when humans will live to an age of eighty thousand years, in the
  city of Ketumatī (present Varanasi), whose king will be the Cakkavattī
  Sankha. Sankha will live in the palace where once dwelt King
  Mahāpanadā, but later he will give the palace away and will himself
  become a follower of Maitreya Buddha.[15]
  
  The scriptures say that Maitreya will attain bodhi in seven days
  (which is the minimum period), by virtue of his many lives of
  preparation for buddhahood similar to those reported in the Jataka
  tales.
  
  At this time a notable teaching he will start giving is that of the
  ten non-virtuous deeds (killing, stealing, sexual misconduct, lying,
  divisive speech, abusive speech, idle speech, covetousness, harmful
  intent and wrong views) and the ten virtuous deeds (the abandonment
  of: killing, stealing, sexual misconduct, lying, divisive speech,
  abusive speech, idle speech, covetousness, harmful intent and wrong
  views).
  
  The Arya Maitreya Mandala, founded by Anagarika Govinda is based on
  the idea of the future coming of Maitreya.
  
  Pali sources say that beings in Maitreya's time will be much bigger
  than during the time of Sakyamuni. In one prophecy his disciples are
  contemptuous of Mahakasyapa, whose head is no larger than an insect to
  them. Buddhas robe barely covers two fingers making them wonder how
  tiny Buddha was. Mahākāśyapa is said to be small enough in comparison
  to cremate in the palm of Maitreya's hand.[16]


~Source - https://en.wikipedia.org/wiki/Maitreya

Here's the thing about the periods of time in different Sasanas of Buddhas. It is a matter of how long the humans live.In the time of our Lord Buddha maximum human life expectancy was 120-125 years.But in next Lord Buddha's era it will be way longer, Therefore that Lord Buddha's Sasana will hold on for a long time.