Yeah... as someone who has blocked a sword before with a shield, timing is basically meaningless. If you wanted to maybe disarm or stagger your enemy, then sure, go ahead, time the movement with their swing for maximum impact, but otherwise... it seems like a forced game mechanic not grounded in any sort of realism, even though its sort of touted as such. Meh.
The only place I've seen it touted as realism is here it has some realism to a point though simply holding up a shield won't do squat because it will be much easier for them to find a way around, plus this is a game and as far as we know we can't position our shield so this is as close as we're getting to realism for now. I already said this but I think realism was not really the
main focus just a way to make the combat more engaging while adding a bit of realism.
The way I think it'll work is
just pressing the block button will bring up your shield, you will have to time it and it will come down after either a few seconds or you press the block button again. To bash you hold the block button instead of just pressing it. From what I've read it seems pretty likely to me and it avoids the situation someone said earlier where advancing on a person shooting arrows would be frustrating when you would have to "catch" their arrows.