Quake II time demo is a good indication of your AVERAGE frame rates. If you want to see a worst case scenario to tell you what the lowest frame rates you will expect to get playing quake II, goto the following url:
This will give you a link to download the crusher.dm2 timedemo map. Extract this file to your quake II directory, and read the readme file, it will tell you where to put the crusher.dm2 file and how to run it in quake II.
To give you an indication, my p2-266 w/6mb canopus pure3d video card can run the demomap demo1.dm2 at 26fps.
When I run the crusher.dm2 map, the game reports 19fps. This is quite a drop, and makes the game slightly choppy to view. To see what your framerates will generally stay around, you need to look at the massive1.dm2 map. This was done by the same person, and shows you what the usual average frame rates will be in multi-player quake II. Goto url:
This will download the massive1.dm2 map, just follow the same instructions for this one as you did for the crusher.dm2 map. My system runs the massive1.dm2 map at about 23 fps.