Tried running the sieve jobs on a CPU, they are quite slow.
Probably 12 hours on a q6600.
Assuming you have 4 jobs running in parallel then it should be possible to do 8 a day.
Around 18K cobblestones per q6600/day
Long term i wouldn't use a CPU to sieve, video cards do it so much better.
However for the duration of the challenge it would be worth doing if someone had lots of CPU's