recentf-keep

recentf-keep is a variable defined in `recentf.el'.
Its value is

(recentf-keep-default-predicate)

Documentation:
List of regexps and predicates for filenames kept in the recent list.
Regexps and predicates are tried in the specified order.
When nil all filenames are kept in the recent list.
When a filename matches any of the regexps or satisfies any of the
predicates it is kept in the recent list.
The default is to keep readable files. Remote files are checked
for readability only in case a connection is established to that
remote system, otherwise they are kept in the recent list without
checking their readability.
A predicate is a function that is passed a filename to check and that
must return non-nil to keep it.

You can customize this variable.