I/O Wait is generally tied to the speed at which the hard drive can run compared to how much data is accessed. As such, I/O Wait doesn't have a standard value that is considered high. What is considered high for a given system is when the I/O Wait affects the server's overall performance. However, consistent I/O Wait over 10% should be investigated, as it could be a sign of a degraded or failing drive.
Please note that temporary I/O Wait elevation above 10% during heavy workload can be normal. This is commonly seen during backups or other tasks that read/write a large amount of files and directories.