-how many frames does it take for a hold to become active?
0. Holds immediately startup as soon as you input the command. There are 3 types of hold dealing damages depending on how accurately you react. Hi-Counter Holds deal the most damage are in the first couple of frames of the hold. Counter Holds deal less damage and are in the few frames after that. Normal Holds deal the least amount of damage and are between Counter Holds and the start of the recovery frames.
-can you hold while at negative frames?
In the context of stun, yes. In the context of recovering from performing an attack that puts you at a frame disadvantage, no.
-when can and can't you hold out of a combo?
Holding within certain combos has become easier in DOA5, though many attacks that cause deeper kinds of stuns than simple hit-stun cannot be held out of reliably in DOA5 as well. Basically, the most consistent unholdable stuns in DOA5 are the new
Critical Stun from a
Critical Burst move, and Jann Lee's


-if you can hold out of stun combos, does this mean i have to learn to mix up my combos?
Generally speaking, yes. Mixing up in Dead or Alive has always been a virtue because of the chance an opponent could easily hold your next combo'd attack. Again though, DOA5 isn't properly fine-tuned in this regard, so it's much easier to connect a launcher from certain stuns (which guarantees a combo) than in previous DOA games.
- generally, am i trying to put up a hold and hoping they make the mistake of attacking into my hold? or is it more me reacting to what they attack with and holding accordingly?
Either way; It's honestly a playstyle choice. Certain gameplay mechanics are built around both scenarios - though with a bad net connection, the first option may seem more reliable (silly enough).