You make me smile, nearly 10 years as solo consultant too. For the past 5 years my "week of work" is 25 billed hours and for extremely hard problems (reworking old complex Fortran thermodynamics code) this goes down to 15h/week. Above that, I cannot sustain the rhythm more than two weeks.

The non-client work is done on an "energy and motivation" basis, so I accept that some weeks are good but I can do nothing two weeks in a row.