Evening,
The default setting for SDRS is 15 ms (vSphere 5.0 Storage Features Part 6 – Storage DRS – Balance On I/O Metrics | VMware vSphere Blog - V… ) In some environments with heavy load and not much cache I have seen 25 ms used to avoid triggering SDRS during backup's and other I/O times. Really it comes down to your environment you have to know your environment and vendor best practices before you can figure out why they set it. Assume that 25 ms was set for a reason since it's not the default. In my environment we ended up with 10 ms and it was great... but it really depends on your environment. Let me know if you have additional questions.
Thanks,
J