Pins are reshuffled every 10 minutes and refreshed from Pinterest every hour.
Endless possibilities: For example, you can stay up to date with pins from people you follow (just log in to your Pinterest account and use pins from the main page). To keep up with what's popular on Pinterest, use the "popular" category. How about some vacation ideas? The "Travel & Places" category is perfect for that.
More ideas: You could start to collect pins on boards specifically for your desktop background depending on your mood, the season, inspiration, vacation ideas, and so on.