We recommend using the cable that comes with the UV+.  If this is not long enought we would recommend 6' or less.  USB can go to a max of 6.4 (5m) but with the high data rate and the need for very quick responses it is best to use as short of a lenght of USB cable as possible.  The longer the cable the longer the time to get data between the UV+ and the computer and can cause issues or the device not to functuion.