SystemInfo.deviceUniqueIdentifier max length?

I would like to know the maximum possible length of SystemInfo.deviceUniqueIdentifier.
Unfortunately i can not find this in the documentation: Unity - Scripting API: SystemInfo.deviceUniqueIdentifier

The reason behind this is as follows:
We want to use this ID as a primary key in one of our database tables, because there will be an index on that primary key the smaller the nvarchar the better.

When inserting a USB flash drive in Windows SystemInfo.deviceUniqueIdentifier changes!