Looks like close to ~30% last year (after seeing ARR fall by 30%). [1] I think higher in prior years.

He claims to have spent only a handful of hours on BCC last year. So spending less than say 20 hours on a project and having it generate ~30% of revenue, passively, seems like a pretty good deal.

Another poster mentioned focus, but that doesn't really seem to be the case here.

Of course all this is conjecture.

[1] http://www.kalzumeus.com/2014/12/22/kalzumeus-software-year-...