I am asking myself if there is a time difference between the events in Thor and The Avengers? Is Loki presumed dead a long time?
Some unknown amount of time passed between the end of Thor and the start of The Avengers. This timeline shows the best timeline of events in the Marvel Cinematic Universe that I know of, and it just has an ambiguous space of time between the end of Thor and The Avengers:
Given that timeline, it would be likely a few months. At the end of Iron Man 2, Tony Stark hadn't begun construction on Stark Tower. That was being completed at the start of The Avengers, so the time spent tearing down and rebuilding that building would be between "Day 4" and the start of The Avengers.
A recent answer indicated a more canonical answer. There's roughly 1 year between the events of Thor and The Avengers.