"For us it is about 8 weeks.   That is time for IS to install the system and do some limited testing. (We (IS) test to make sure the batch jobs, still run, not worrying about the outputs validity.)  Both HR and Payroll do their testing of the windows, adding employees, moving employees from one location to other locations, changes in benefits, etc.  The last big item is to run the current system and the new system in parallel for two payroll cycles, and to fully test out the new system."

"This is what we do.  How long and how much effort you and your team puts into testing 6.4 is up to you and your team."

In addition I [Denton Harryman] responded with the following recommendation about additional testing which should be performed:

In addition to testing 6.4 to insure your current use of emPath will continue to work, I recommend you test add more than 12 tax codes and more than 42 Pay/Deduction codes to a payroll group.  The reason I note this is because the number of tax codes and pay/deduction codes, which can be placed on a tax group, has been significantly increased in release 6.4.  This is great news especially for those paying in Quebec. 

As part of this major improvement many of the batch programs were modified. 
 
Also, if you have custom programs or reports which use the ETF2 table you will need change these programs to take advantage of the increased number of tax codes and pay/deduction codes.
 
Given several clients have implemented 6.4 I suspect you will not have problems testing your current use of emPath on the 6.4 release.  Given the internal testing NOW has performed I suspect you will not have problems when you use the extra tax codes and pay/deduction codes on the tax group records. 
 
I hope this information helps.