Yeah, I'm of the "it depends" view too.
My dogs all know the words and the actions, so they are fluent, and as Sabine pointed out, that is a factor.
Then there's the clearness of communication issue. Did my dog hear me tell him to sit? Did I have my dog's attention when I gave that direction, or was he distracted? If he was distracted, I will repeat the request when it becomes obvious to me that he just didn't hear it.
If he received the instruction but chooses not to act on it, I will stand there for a pretty long time. I might use my body language to suggest what I want from him, or -- with a Border Collie -- a meaningful look. And after a minute or two I might ask, in a conversational tone, "what happened to your sit? Is it broken?" Or I will use a phrase that the dog knows. When I say to Rowley "what do we do?" he sits in front of a door or gate and waits for me to open it. If he doesn't offer the sit/wait of his own volition, I ask him 'what do we do?'
I don't think it really matters too much ... to me, it's not a test of anything, it's okay to remind the dog what was wanted. I think the main thing is not to repeat the instruction ten times in a minute.
My obedience trainer says I make her crazy because I repeat commands with Rowley, but in herding, directions are not static and the dog is free to get up and move on the sheep after a 'lie down', so I train him a little differently because of that. Those are more reminders than commands, though. And FWIW, I hate the word "command" in this situation, it makes me feel like I should be wearing camo and boots and a combat helmet.