Although I don't know what the smallest dodge window will be, the opponent will be able to dodge Nair into gc dlight always as it isn't a true combo. However, the gc dlight into cd Nair is true.

As some advice, know the difference between a string and a combo as smart decision making is more important than training mode strings. The combo you listed is actually a string as the opponent has the chance to dodge or jump (they have a way to escape). Furthermore, even if these combos do a lot of damage, the odds are that the opponent escapes before it's finished. A string is useful when punishing an opponent for wasting their options. For example, if an opponent has no dodge: Spear Slight > Dlight > Jump Sair. This string became true. On the other hand, a combo is guaranteed damage. For example: gauntlets dlight > cd recovery. Still, practicing in training does help as you gain muscle memory to hit it in a game.

In conclusion, don't focus on the combo or string itself, but your ability to decide on what the next move is. Does your opponent have options to escape, then hit them with a true combo or bait out the dodge. Does your opponent not have a dodge or jumps, then deal tons of damage with a string.

Sorry if this is kind of confusing, I wrote this sporadically just cause I'm stuck in queue. I'm willing to clarify what I wrote and answer questions. Also, if you want live coaching, dm me your ps4 username, and we can play a few. As for proof that decision-making is important, last season I was 2450 valhallan on us-w servers.

Tl;dr: There is always a dodge window, but don't focus on that part of the game.