While I can only attest to the experience from the Consumer side of the equation. I occasionally personally run a 'On Demand Scan' (Not under Command Prompt) I might add. My results being it basically takes the same amount of time as similarly allowing a 'Scheduled Scan' to take place.
I experience no interruptions what-so-ever in performance, etc. Just thought I would add that...
Running 'McAfee Total Protection'
OK I confirmed that the on demand scan will start when the computer is idle, and will continue running until completed, or the specified time has elapsed. So it's not like the task will stop running when the computer stops being idle.