I would think the default is calculated based on disk size, not sure however.
I currently have 5 GB and it's getting full. I'd say 10-ish should be fine. Of course, it depends on usage, for example, kernels are 150 megs a pop and old ones are not automatically removed and new ones keep pouring in (unless you remove the linux-image metapackage which I did because I use a custom kernel anyways).
