The Qaylulah is a sunnah, it is a nap or rest (even without sleep) taken midday or when the sun is at its peak. It is usually taken before Zhuhr prayer unless the prayer is held early (in the beginning of its time) in which case it may be done afterwards.
Imam Bukhari related in his Sahih that Sahl ibn Sa’ad, may Allah be pleased with him said, “We, [the Companions], would not take the qaylulah nor eat [he morning meal] except after Jum’ah.”
In his commentary ‘Umdat all-Qari, Imam Badr al-Deen al-‘Ayni explains that this indicates they were preoccupied with getting ready for Jumah and the prayer which caused them to delay their meal and nap from its normal time before Zhuhr prayer to after Jumu’ah.
The length of this nap is not specified, but it is a nap, not a lengthy slumber.