While it might seem like a simple question, unfortunately it's not, as it depends on your meaning of authenticate. The time it takes to authenticate a set of credentials against a domain controller, is a pretty simple process, i.e. issue a bind statement and measure how long it takes. NetTools provides an option to do just this see the LDAP Performance option for details. However, measuring how long it take a workstation or server to complete a logon process is a little more complicated, as the logon process is made up of multiple steps completed, both locally and over the network, with multiple requests to the domain controllers, with each one authenticating each time and dependent on the other factor both on the network and local workstation\server that can impact the performance.
Can you provide a bit more information on the problem you are investigating?
Gary.