The chokes don't happen immediately, you have 30 seconds or so to start feeling light headed and realize that things are getting dark!
Are you talking about a choke that closes the carotid or one that crushes the airway? Cause 30 seconds is a *long* time in my experience. 5-10 seconds is what I'd normally say when I've seen it done well.
There's a good article here I just found surfing around on the topic.
Article on choking in judo by an MD.