Disk Latency

a) Exchange :

http://technet.microsoft.com/en-us/library/aa995945(v=exchg.80).aspx

The average Read latency to the transaction log drives should be below 20 ms. Spikes in read latency should be under 50ms. 

b) Core:

http://blogs.technet.com/b/askperf/archive/2010/11/05/performance-tuning-windows-server-2008-r2-pt-2.aspx

Since modern disks are typically rated at well under 10 ms random access time, any number much higher than that is problematic. 

c) Core:

http://blogs.technet.com/b/askcore/archive/2012/02/07/measuring-disk-latency-with-windows-performance-monitor-perfmon.aspx

You have probably heard general statements about what are acceptable disk latency measurements: “Less than 10 milliseconds is good and more than 20 milliseconds is bad”. Although these rules of thumb are used to simplify analysis, they do not apply in all cases and may lead to incorrect conclusions.

Important consideration:

The time the IO spent in queue is added to the disk latency in perfmon.