The gift of lands and / or a manor to an individual technically lasted 'forever'. 'Technically', because the 'gift' was conditional upon the continued performance by the grantee of the service specified. When the person died his lands passed to his heir according to established rule. Wills and testaments were rare, normally not straying from rules regarding land, and likely concerned only movable possessions, cash legacies, and the like.
What passed to the heirs by rule was the lord's landed estate. What went with the estate were the obligations attached to the lands, or service, often the whole or part of a knight's fee. It would make no sense for a knight's fee to be inherited on its own, because there would be no property estate to produce the income that provided the service fees.
The Lord's Manor and premises passed according to set criteria: If there were one or more sons of any age, then the eldest was the heir. If the eldest son died, then the second son inherited, and so on through any and all successive sons. If there were no sons, then all the daughters shared the estate equally. While the Lord was alive, the eldest son was known as the 'heir apparent'. If there were no children, then the same rules applied, in that the next younger brother of the deceased became the heir, or that brother's son or daughters, etc. This was expressed as, for example, 'the manor to the eldest son, John, with remainder to the second son, William'.
From early times the king appointed 'escheators' for specified territories, often one or more counties of England. When a landed lord died, the king sent a writ to the escheator, instructing him to hold an 'inquisition post mortem' or IPM, for the deceased, to establish what lands he/she held and by what right. As an example of a simple finding, the escheator and his jury of local residents might find: 'When he died, John Mallory held the manor of Welton by service of one knight's fee. His heir is his eldest son, Robert Mallory, aged 25 and more.'
If there were no valid heirs, or the escheator found that the deceased person did not validly hold the lands, then the estate was 'escheated', or handed over, to the king. If the heir was under age, then the estate was placed in a wardship, held by a 'reliable' individual, until the heir came of age.