Humayd narrates from Sayyiduna Anas (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) took an oath of ila from his wives, and his (peace and blessings be upon him) foot was injured, so he (peace and blessings be upon him) stayed in an upper room for 29 nights, then came down. They said: "O Messenger of Allah (peace and blessings be upon him), you had sworn for a month." He (peace and blessings be upon him) said: "A month is sometimes 29 days."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الإيلاء / 15236
Ata' (may Allah have mercy on him) narrates from Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) that in the days of ignorance (Jahiliyyah), ila used to be for one or two years or even more, but Allah, Lord of Might, fixed its time at four months, if someone wishes to take an oath of ila. In the narration of Yunus: Whoever takes an oath of ila for less than four months, that will not be ila.
Ata' (may Allah have mercy on him) said: If the husband takes an oath of ila while the wife is still in her parents' house and has not yet been sent to him, that will not be ila.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الإيلاء / 15237
Ibn Tawus (may Allah have mercy on him) narrates from his father (may Allah have mercy on him) concerning ila (al-ila') that the husband swears not to have intercourse forever, or for six months or more, or for more than four months, or something similar.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الإيلاء / 15238