The PowerMill toolpath statistics calculate linear length times speed max speed, the same with rapids. This does not take into account machine acceleration and deceleration.
Find your machine's average runtime.
If your machine runs for 2 hours when PowerMill statistics tell you 1 hour, then you can double the PowerMill statistics.
Use the machine's average to calculate runtime.
Once this number is captured, you can use it to find a more accurate runtime for all toolpaths. Take the toolpath statistics estimated runtime and multiply it by the machine's average.
Add a runtime multiplier to your post-processer.
You can also request a post-modification to multiply the toolpath and program runtime in your output NC code. This will output a more accurate runtime for the operator.