So how to know exactly how long a stun last? Does the "advantage" in the first page of the move details tell you how long a stun last?
Yes. It shows how many frames the stun will last. In general, the higher the advantage is, the harder will it be to escape from it. If you have a follow up move that will reach the opponent and will hit within a frame window that is at least two frames less than the stun advantage on fastest stagger escape, it is most likely guaranteed to hit.
Another questions is what kinds of stun if opponent gets can not be hold and/or SE? I know some sit down stun has a guaranteed follow up even if opponent SE. For example, the stun when opponent gets while they hit by the obstacles,can not be hold or SE.As for Limbo Stuns and faint stun, although they can be SE, but can they be hold? Also after the opponent SE, can they hold in sit down stun?
Being able to hold in stun depends on the stun type itself and the advantage the opponent will get from it. The latter is different for each stun and character so you have to check it in the training mode. As reference for how the different stun types look like, you can check out this video made by
@Allan Paris:
http://www.freestepdodge.com/media/doa-stun-types.6874/media
And to give you a little, written down summary about the most important characteristics of the different stun types:
Lift stun - Usually gives you a high amount of advantage, meaning these are very deep stuns. Follow-ups cannot be hold if no slow escaping is done. Even with slow escaping, the hold will come out late.
Crumple stun - Nothing special, follow-ups can be countered. Sometimes, follow-ups that are highs will miss due to the stun animation.
Falling crumple stun - Like the crumple stun, but the opponent will fall to the ground if no follow-up is done.
Trip stun - Caused by lows, offers a moderate amount of follow-ups which usually can be all avoided on fastest slow escaping.
Stagger stun - Nothing special here.
Faint stun - Cannot be slow escaped and follow-ups cannot be hold. The stunned player will fall to the ground if no follow-up is done but the options are limited due to the animation.
Limbo stun - Cannot be slow escaped and follow-ups cannot be hold due to being backturned. However, as soon as the angle is a bit off (meaning the agressor is not directly in his opponents back), it might be possible to counter out of it. Otherwise, the stunned player will fall to the ground if no follow-up is done.
Sit down stun - In general, follow-ups can be slow escaped and then blocked or countered. However, some characters like Hayabusa also have sit down stuns that guarantee unholdable follow-ups.
Slip stun - This stun is caused by lows that will hit on counterhit on slippery surfaces like ice or water. It cannot be slow escaped and follow-ups cannot be countered. Available options for moves that can be used as follow ups are usually very limited due to the stun animation.
Face stun - Nothing special here, the amount of frame advantage from the stun is moderate. Countering or slow escaping is possible.
That's about it. I wrote this out of my mind so if you or anyone else notice something incorrect, just tell me and I'll change it our cut it out. Hope it helps you.