No idea. Depends on your kernel architecture (monolithic or microkernel?), scope of your OS (generic desktop, server, game engine, embedded, ...), and many other factors that are also bound to change over time.
As for "pure segmentation"... what's your compiler of choice? I'm asking because GCC does not support segmentation, but assumes a flat memory model...
Edit: I was a bit at a loss for words, but now I found the metaphor: Your question is like asking us, "how much money should I take with me for my holiday?", but not telling us where you're going, how long you stay, and what your acount balance is. And even then we couldn't really give good advice...