Description of problem:
ATM it is only possible to set a general metadata expire but it would IMHO make more sense to be able to configure this per repository. So you could i.e. set the expire value for update-repositories to lower value than for normal repositories. This would safe bandwidth on the one hand and on the other updates could still be found and applied i.e. once a day.
Version-Release number of selected component (if applicable):
$ rpm -q dnf
This is already possible, we just documented it wrong. This is addressed by the 8bdb2a0 upstream commit. The documentation will be released with dnf-0.4.15.
dnf-0.4.15-1.fc20 has been submitted as an update for Fedora 20.
dnf-0.4.15-1.fc20 has been pushed to the Fedora 20 stable repository. If problems still persist, please make note of it in this bug report.