I am aware of the different yugas and their different length. As far as I know the time lenghts stand in a ratio of 4:3:2:1. I believe that the ratio is always the same, but my question is whether the absolute time depends upon in which mahayuga we are in. Moreover I have another question, do the yugas after descending from Satya Yuga to Kali Yuga ascend in reverse order to Satya Yuga or does Satya Yuga immediately follow Kali Yuga?
'Yuga' technically means 'age' or 'era'. In Vedic metaphysical cosmology, there are 4 Yugas which consist of a fixed number of years, based on the magnitude of prevailing spiritual righteousness, or Dharma.
The collection of all four yugas is called a Chaturyuga. One day of Lord Brahma consists of 1000 such Chaturyugas. This time is called "Kalpa". During this time 14 Manus come and go. Each Manu is created by Lord Brahma Himself, who assigns the Manu with the task of continuing the process of creation by creating generations of mankind called 'Manvantara'. One Manvantara is approximately 71 Chaturyugas. After the Manvantara, Lord Brahma creates another Manu, to carry on the process of creation and procreation. In this new Manvantara, Lord Vishnu takes on a new form, and a new Indra and new Saptarishis are appointed. The current form of Vishnu is the famous four-armed figure having a discus, a lotus, a conch and a mace in each hand. The current Indra is Purandar. The current Saptarishis are Kashyapa, Atri, Vashista, Vishvamitra, Gautama, Jamadagni and Bharadvaja. Later on I will tabulate the other Saptarishis in all the 14 Manvantaras of this present Day of Brahma or the present Kalpa (1000-Chaturyuga-period).
After 1000 Chaturyugas have passed, Brahma's Day is over, which leads to Brahma's Night. During this time, there is no creation as Lord Brahma takes rest. It continues for the same amount of time as Brahma's Day. After the night is over, Brahma commences a fresh creation. The night of Brahma starts with 'Pralaya' (annihilation) and ends with 'Srishti' (creation/manifestation).
As far as your second question goes, I am not aware of that because different people have different assumptions and theories. For example the great astrologer and master Sri Sri Yukteshwar Giri, the Gurudev of Paramahansa Yogananda, has stated that Kali Yuga gradually ascends to the Satya in a reverse order. He primarily said that because the principles of mystic development would be known by a more scientific insight in the Kali Yuga. For example the working knowledge of telepathy, telekinesis, psychokinesis, teleportation, astral projection etc.
But from the absolute point of view, Kali Yuga will last for 4,32,000 years. All scientific developments will take place till this limit of 4,32,000. Then a new Satya will emerge after Kalki Avatar.