The maximum length of a standard USB cable (for USB 2.0 or earlier) is 5 metres, or about 15 feet. There are a number of USB extenders that will allow you to use a USB connection up to 150 feet, but the cost of these extenders exceeds $150. For a lower cost you can purchase IP cameras to replace the USB cameras you have.
A full list of supported IP cameras in WebCam Monitor are available here:
https://www.deskshare.com/faqproduct.aspx?ID=12#10
