By using an app that does incremental back ups. The first time you run the back up the app will make a complete copy of the Library. Thereafter it will update the back up with the changes you have made. That makes subsequent back ups much faster. Many of these apps also have scheduling capabilities: So set it up and it will do the back up automatically. Examples of such apps: Chronosync or DejaVu . But are many others. Search on MacUpdate or the App Store.
I use Synk Pro to incrementally backup my library. I have it set to check and do the backup when iPhoto is closed. That keeps the activity down while I'm using iPhoto. When I close iPhoto Synk Pro immediately scans the two libraries and makes the backups necessary.
But any incremental backup app will make the process short and simple.