We're in the realm of speculation here, not a lawyer etc. 5 seconds is a fairly decent amount of time if you count it out. Plenty of time to register what you're seeing.
1 second (and below) it might be possible to argue it's too short to register the advert consciously, making it a subliminal advert. Again I don't know if this is true and I'm completely making this up. It'd be interesting to find out the answer.
A second is enough time to consciously register an ad. To prove it, they could run a bit where someone shouts "50 percent?!" with a picture of an iPad and "Tap! 50% off". I'll bet people start tapping by the second or third impression (even though it's obviously a scam of some kind).
1 second (and below) it might be possible to argue it's too short to register the advert consciously, making it a subliminal advert. Again I don't know if this is true and I'm completely making this up. It'd be interesting to find out the answer.