Simak al-Hanafi (may Allah have mercy on him) says: I heard Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) say: When the beginning of Surah al-Muzzammil was revealed, the people used to stand in the night prayer like (their standing in) the month of Ramadan, until the last part of Surah al-Muzzammil was revealed. Between the revelation of the beginning and the end of Surah al-Muzzammil there was an interval of about a year.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4640