tramp-completion-reread-directory-timeout

tramp-completion-reread-directory-timeout is a variable defined in `tramp.el'.
Its value is

10

Documentation:
Defines seconds since last remote command before rereading a directory.
A remote directory might have changed its contents. In order to
make it visible during file name completion in the minibuffer,
Tramp flushes its cache and rereads the directory contents when
more than `tramp-completion-reread-directory-timeout' seconds
have been gone since last remote command execution. A value of `t'
would require an immediate reread during filename completion, `nil'
means to use always cached values for the directory contents.

You can customize this variable.