I'm creating a partition (from backup) with dd and need to know if there is a need of some sort of sync/timeout before I can access it. As I know data may be cached in dirty pages. But what if I read/write a file or device that has some part of its data in cache. Will I get/overwrite most recent data (from cache) or only data that has been flushed.
You'll be pleased to hear that none of that is relevant. You (presumably) restored via
/dev/sdb1. That same device is what you're going to mount, so it sees the same content that you wrote.
What wouldn't work is restoring the partition table by writing to
/dev/sdb and expecting
/dev/sdb1 (say) to magically spring into existence. For that you'd need to run something like