On Sun, 17 Dec 2017 22:43:17 +0100, Emanuel Berg
wrote:

It seems after putting on a new chain, it turns
really slack after only 2-3 weeks. When you
then restraighten it, it remains tight
considerably longer. Is this so or does
intuition play a trick on me? In general, how
many bike hours does a chain last?

That's sort of asking "how long is a piece of string" :-)
Bike chain, how many speeds, runs inside a chain case, how fast, how
much power transferred, new cogs or old, lubricated, how frequently,
etc., etc.
--
Cheers,

John B.