How can I list in long format all files (located in a directory) which belong to me (rights) and were modified more than 7 days ago?
Use the find command with an argument for files owned by a given username and modified over a certain time.
find -user username -mtime +7
EDIT: Optionally (if you use GNU find, mandatory if you use other implementations) you can add a directory location to search in like this:
find /path/to/search -user username -mtime +7. GNU find assumes you want to search from the current directory if you do not specify a path.
ls -ld *(u$UIDm+7)
m are globbing qualifiers to match files by owner and modification time respectively.