Size of Xenforo Session cache using Xcache?

Hi, what is a normal Xcache memory usage for a fairly big website when it comes to the session cache?
I don't quite understand how it works, it caches also guests; why is that?

We have over 100 000 members, over 1 million messages & almost 200 000 topics.
At any given time there may be around 1000 unique users on the forums (including guests)

With a 32MB var cache in Xcache we fill it up quite quickly, but what's a sensible value to set it to?
There must be a point where more processing is actually required for making the session caches when the hit-rate is very low?

If I look in our Xcache var data cache info right now over 95% of sessions have only one hit.
And it's run out of memory, but with my question above in mind is it even sensible to increase it more?
Or should we just turn the session cache off completely?