This isn't necessarily what is going on in this thread currently, but it's a good thing to bring up for this thread.
What is our goal for shield stun?
Right now people are just saying values that they like, but we need to define what our goal is. I have a pretty good idea of what I am personally striving for, and I will share it with you guys to see what you think.
1. Weak hits
Weak hits should have a decent amount of shield stun. Specifically jabs. There should be enough stun that one could not simply immediately shield grab a jab (which is why it's useless in Brawl) so there can be a jab game. Thankfully, without buffering, one cannot simply press the grab button and get a frame perfect grab after a jab. Basically, the stun that I would see as ideal is for a jab to be able to stun the shield for most of a full jab animation. This means that jabbing would be a mixup game.
Here is an example. I land an aerial on my opponents shield, and I am able to jab afterwards. This begins a mixup. At this point if I hit with the jab I would go straight into a grab (yay for hitstun). If I hit his shield, most of the time I would also go for a grab, as most people these days in Brawl aren't familiar with the jab game that melee had. If my opponent was familiar though, he could simply mix it up himself, shielding when he thinks I will jab, try to make jabs himself, performing an aerial if he thinks I'm about to go for a grab, or what have you. In any case, in high end play an elaborate dance would begin, one that I really miss from the melee days.
2. Average aerials
Average aerials should have some stun, but should have a decent possibility of being shield grabbed. These are typically combo starter moves, so shielding them should be the counter to them. Granted if you land one of these you are rewarded typically with a string of hits. Risk/reward, you get the idea.
3. Strong aerials
Strong aerials should have a good deal of stun. They should have enough stun to where shield grabbing could not occur; however, it doesn't put the defensive player in too bad of a position. Strong aerials, in my opinion, should force a reset on your opponent, in which once you land both players are on roughly equal footing. This actually ties in with my earlier example.
I am playing Falcon. I get inside my opponents reach and land a knee. He shields it. I land in front of his face, but both my attacks lag and his shield lag has ended. We are now in roughly an equal position barring character differences. The main point of this is that Falcon has a great jab game, and I could then bring in jabs and start the jab game going.
4. Drills
Drills should act in a similar way as a strong aerial. Since they have little lag most of the time the amount of shield stun they have should last roughly the same length as the aerial's attack lag.
These are ideal goals I'd like to see come into play. Now I know that you can't make all of the strong hits work exactly as I've stated, and you can't get all of the drills to work that way as well; however, if we can get it to where most characters could fit into this sort of ideal goal, then we'd be good to go. Most characters have either a big power hit or a drill, and a lot of characters have good close range options. Those who don't have close range options are typically not good at in-fighting, so their game revolves around preventing their opponents into getting into such a positional advantage. So yeah, I think if we try to make it work with guidelines such as these we can get a lot of depth out of a single change to the Brawl engine.