The length of time that Parakeets will live for depends on the species of the parakeet.
But on average Parakeets have a lifespan of as low as 5 years and as long as 15 to 20 years with 15 years being the average length of time that Parakeets live for.
My Grandma had a Parakeet that lived for 21 years but that is rare and it only lived 1 year over it's expected life expectancy.
A life expectancy is just really a guess as to how long the parakeet will live and parakeets just like humans and dogs can live a bit longer or shorter than the life expectancy.
But if you get a parakeet you should expect to have it for at least 15 or 20 years depending on the age the parakeet was when you got the parakeet.