That would depend on how much new information there was for it to write. For example, my initial 'sync' took about 2.5 hours. This was for about 3.5TB. If you data changes very little, the 'sync' will only take a couple of minutes. If there is no change, it will take a seconds.Is there a limit to how often it can be run? Can I set it to run hourly or every 3-4 hours, or would that be detrimental to performance?
It does use some CPU. I have an overclocked 6600k. It uses about 8% of CPU (I assume that's only one core, but I haven't checked.) But, that is only while the 'sync' is running.
So, after your initial 'sync' you could theoretically run it hourly without issue. On a server, I wouldn't think this would cause any issues.