@User: I have not worked on one. Once we have the device models installed, it's a pretty simple process to make one. The instructions are on the website. Basically, you just need to run an inverter through ngspice and read off rise times and fall times. Note that due to the different transistor types used in some of the standard cell sets, there is not one ".prm" file, but several.

@Tim Edwards: thanks, i'll make some prm files once the device models are available.