Manage Your Time Machine Schedule
Apple designed Time Machine with the expectation that most people will back up either to a local drive that's always connected, or to a network volume that's always available. However, if you travel with a laptop, or if for any other reason your destination disk isn't always available, don't worry: Time Machine still works fine.
Time Machine runs hourly, but if the destination disk happens to be unavailable, it simply ignores the problem and tries again as soon as the volume reappears. So, even if you have the external disk connected (or the network server mounted) only intermittently, Time Machine simply waits to back up your files until it's available.
The only problem is that, in the meantime, your files won't ...