Blood flow through the SVC is around 2L/min---some variation with fluid status,patient size,etc. but it's fast enough that you probably don't need to wait a full 5 minutes. Turning off all pumps is indeed crucial as is a good flush and appropriate waste. Here's a reference:
Journal of Infusion Nursing
Volume 26(5), September/October 2003, pp 285-293
Drawing Blood Samples From Vascular Access Devices: Evidence-based Practice
Frey, Anne Marie RN, BSN, CRNI
A link:
http://www.hadawayassociates.com/
Look under the Clinical Articles section for the one on blood sampling.
Another link:
http://www.iv-therapy.net/node/702