I'll add in my own two cents: I know for a fact that Robin's first hit of jab has enough ending lag for him to not be able to shield the sub. I'd imagine that's the limit, but I don't know anything solid about frame data =/ For another comparison, Mac cannot shield if he Ftilts either.
I think an important thing to remember is that the Substitute isn't just for show. It can actually cause some multi-hits to suffer quite a bit of hitlag (I believe the hit that activates the counter always suffers hitlag). While this is good in some cases (MK Fair, Greninja Bair, Mac Ftilt, etc.,), it does result in the slower multi-hits being extended way too long.
I think I'll go ahead and test to see if anything faster than Robin's Jab also works and edit this post with said results.
Edit: Fastest jab I could punish before a shield comes out is DK's (for reference, it's faster than Duck Hunt's and Wario's I believe). Of course, any character that continues the jab combo is gonna hit Greninja out of the attack, but having a frame of reference for how slow a move has to be for Sub to connect is nice.