5 frames.vulnerability, 24 frames intangibility starting at frame 4.
http://smashboards.com/threads/air-...-the-auto-cancel-window.373035/#post-17770416
Let's see, you have 35 frames to recognize the animation (earliest being the frame where the animation is distinguishable), react (12-15 frames), choose an attack (hopefully you have one ready), and have the attack reach its hit frames (minimum 3 frames usually). That's plenty of time to punish, as long as you're closeby, ready to react, and don't choose a very slow attack. In some situations or with some characters you might have to stay close enough to be in harm's way if you want to punish in time, though.
5 frames of vulnerability doesn't mean you have a 5 frame window to input the punish. Say your attack has 3 hit frames that will be in range. That means the hit frames can start two frames before and still hit, for a total of a seven frames window. Imagine trying to fit a 3-square lego block onto a 5-square one. There are 7 different ways to connect them. A seven frame window is easy enough, and that's only for 3-relevant-hit-frame attacks like Marth's fair.
There's nothing particularly bad about the air dodge based on these facts alone. It must recover quick enough that the air dodge actually helps the player get away. It must last long enough that the opponent has time to punish it on reaction if they bait it. It needs to induce landing lag in a way that its drawbacks are not avoided by airdodging close to the ground. It needs to have a fast enough startup that it's a better purely defensive option than attacking. After scrutinizing it, I don't think the air dodge could have been designed any better than it is (except if it gave you momentum ^_^). It seems to perfectly fulfill all those purposes.
If you're annoyed about people easily getting out of combos, blame the hitstun.
And if you try to deplete shields, you get punished. Practically everything is unsafe, and they can just regenerate during the punishment. And safety requires you to be spaced or be fading away with an aerial, where the opponent can easily back off.
Shield-poking can't be reliably used as an offensive strategy unless players both fail to punish and continue to use only shield as their defensive option. You need be heavily outplaying your opponent with projectile+attack setups and delayed string attack mixups that encourage sitting in shield, and you also need to reasonably threatening to cover roll-though, and lastly have them near the edge so they can't just roll away. All those mixups need to be in your favor just to get an extra mixup that's not even that rewarding if the opponent knows to shield-tilt in safe places.
Shields are clearly designed to handle four player free-for-all, strong enough to defend but still punishing excessive shielding in the more chaotic environment where multiple players, items, and stage hazards can be hitting you.