Rephrasing of the question slightly to address some issues.
Is there are standard way of measuring load times for data, specifically sybolised data, within ArcMap?
In particular, we're comparing load times of different data formats (shapefile, SDE, file and personal geodatabases) across the network for the same (large) sets of data. We're also aiming to test speeds between ArcGIS versions.
At this point I've started hacking together a simple VB script to do some of the timing without having to sit there with a stopwatch as @matt-wilkie suggested. The script (thus far) can be found at http://textsnip.com/8912ac/vb. The script was written in ArcGIS 9.3 but works in 9.2 as well.
To use the script, copy the VB script to your mxd, and add two buttons, called "LoadDatasets" and "SymboliseDatasets". The LoadDatasets button allows the load of one or more feature classes or layers, and times the load. SymboliseDatasets checks the number of layers in the ActiveView, and if there are none, calls the load dialog (but doesn't time it). Once layers are added then the SymboliseDatasets button will symbolise all layers into 10 quantile groups based on their FIDs.
EDIT: Appears I have fixed the issue of timing the rendering by adding a DoEvents after the ActiveView.Refresh
In the meantime if anyone wants to pick this script up and modify it to make it more useful I'm happy to set this question to community wiki.