You can either use a timer to trigger builds, or you can poll the code repository for changes and build if there were changes.
It can be useful to use both methods at the same time:
Git repository polling can be used most of the time so that every check in triggers a build.
Nightly builds can be triggered, which are more stringent than continuous builds, and thus take a longer time. Since these builds happen at night when nobody is supposed to work, it doesn't matter if they are slow.
An upstream build can trigger a downstream build.
You can also let the successful build of one job trigger another job.