Have you tried something like incrementing a variable in the DoUSBin loop, and exiting once it reaches a certain value?

DoUSBIn:
USBBufferCount = USBBufferSizeRX ' RX buffer size
USBService ' keep connection alive
X = X + 1
PAUSEUS 5000 ' 5mS delay
IF X > ExitVal THEN JumpOut
USBIn 1, USBBuffer, USBBufferCount, DoUSBIn ' read data, if available
JumpOut:
Return