A very rough estimate would be around ~1TH/s.
I have been keeping an eye on the hashrate of some miners on pool, which I suspect to be botnet miners.
I can tell you that I have seen multiple suspected botnet miners reach 60GH/s.
Remember that botnet operators don't pay for electricity. So if a 10000 nodes botnet is idle, better make it generate 10GH/s (assuming 1MH/s for each host with a stealth mining mode not using all the CPU power) than sit doing nothing.
You can go with the estimate of ~3MH/s.
From my analysis, I concluded that number to be the general hashrate.
I honestly don't think so, maybe back when CPU mining was the thing, but now they'd have to infect people with certain model's of ATI GPU's or mining FPGA's to amount to any amount of hashing power.
The big numbers of infected computers compensate the low hashrate.