Time taken for species to become endemic
From wikipedia > endemism:


  Endemism is the ecological state of a species being unique to a defined geographic location, such as an island, nation, country or other defined zone, or habitat type.


There is therefore no time component to it. If a species is found only in Canada, then it is endemic to Canada. If it is found only in your garden, then it is endemic to your garden. No matter the duration in which the species has been found in this specific location.

That being said, in conservation biology, it is common to oppose endemic species with invasive species. An invasive species

Wikipedia > invasive species


  An invasive species is a species that is not native to a specific location (an introduced species), and that has a tendency to spread to a degree believed to cause damage to the environment, human economy or human health


As the linked wikipedia page explains the concept of invasive species (or if you prefer the concept of non-endemism) is open to a lot of critics and is in no way well defined. There is no time limit under which we would stop considering an invasive species as invasive. It is rather the ecological impact of this species that will people to call it invasive or endemic based on purely subjective notions.