This one is a bit over broad as knighthoods varied based on different countries and cultures. Additionally, it would also depend on if the title of "Knight" is referring to an actual warrior knight, or a general title of nobility, both existed. In England, for example, knighthoods could expire if one did not pay for the "upkeep" necessary to hold the title and it could also not be passed down in some cases. Some knights were barely considered royalty or soldiers and were able to buy the title.

If you could narrow this down, it might be a bit easier to answer based on time period and culture.