This will last for 7 years. But one year for him will be like 20 for you. (Ibn Hajar al-Haythami, Al-Qawl al-Mukhtasar fi `Alamat al-Mahdi al-Muntadhar, p. 44)
In one hadith is reported that one year for Hazrat Mahdi (as) is like 20 years for other people. Hazrat Mahdi (as) will do very quickly what people had been unable to do for many years. He will find very quick, intelligent, wise and pasting solutions to all matters.