So far, I think it's a good idea to run cron every minute and then adjust which tasks I want cron to trigger and when within Moodle, i.e. in Administration > Site administration > Server > Scheduled tasks
Especially for things like automatic course completion and awarding badges, I think users expect something immediate so even a < 1 minute delay can still be too long.
It's also nice for groups to get frequent updates of forum posts on the occasions when users tend to be online at the same time. It's somewhere between asynchronous forums and synchronous chat.