Note: when I say hands of a human I mean that a person can do something to slow down or hold an order. the answer is probably never for Nasdaq. in the past SOES orders were able to be held for 30 seconds. but all the ecn's and supersoes are electronic. There is one caveat - the AMEX. If the AMEX is showing the best bid or offer I believe there is a chance an order can get routed to them. They - probably by design - have poor systems and orders can get held. I'm not sure if IB is routing US stocks there - if so it would be a last resort. There was another thread on AMEX execution which I didn't fully read and I haven't discussed it with our US office. Thus others may want to comment.
I had a stock just last week that I had a buy stop on that should have triggered(I thought) and gone out . The order was not activated or sent. The IB rep said that because the bids were from AMEX and AMEX does not execute electronically that it was not a valid order route so was not activated. The thing that was interesting was that it was a Nasdaq stock. The thing that worries me is with a market order the price could get far away before another exchange triggers the stop....I know , use a limit.