Poudriere works with a preconfigured cache size for ccache.
To check the current size, see /data/ccache/ccache.conf
To check the usage ratio, chroot into one of the (running) build jails or start one using poudriere jail -s -j jailname
There, just enter ccache -s
If you see the cache usage is anywhere close to 90% you might want to raise the limit.
In my case I've changed it from 1.1GB to 4GB by updating the ccache.conf.
As for the stats, you can see the hit rate (before raising the limit) was pretty exactly 25%.
It doesn't mean an exact 25% speedup of the whole poudriere run, but at least 25% of the compiler calls were pretty instantaneous.